It is Cricket 101: the first over should be bowled by whoever has the most chance of getting a wicket in it. But Twenty20 inverts the sport's conventions at every turn. And so, rather than starting the innings with a leading bowler, some teams are instead entrusting the opening over to a bowler regarded as the sixth or seventh best in the side, often an offspinner or medium-pacer.

In this season's NatWest T20 Blast, Darren Stevens has opened the bowling five times. His wobbly medium-pace in these overs has conceded, respectively: 6, 6, 6, 4 and 7 - a total of only 29 from 30 balls.

And yet, however impressive these figures, on the past four occasions Stevens has not been asked to deliver another over after his frugal first. He has become a curious breed: a specialist opening bowler, excellent in the first over and yet not deemed deserving of overs thereafter.

"It's easier to bowl in the first over as the batsmen are more likely to have a look before they get set and play their shots," Stevens says. "I am aiming for the top of off stump, trying to get wickets. I want a slip in place because I know I can swing and seam the new ball. I try and hit the seam, going in or away from the batsman which is easier when the ball's harder and there's a more pronounced seam."

In the nets Stevens practises specifically with a new white ball to hone his role. While he is bowling at a time of the innings ideally suited to his skills, his captain, Sam Northeast, is getting through the opening over on the cheap with a bowler not normally among his leading five.

"That first over is a bit of a sighter almost," Northeast says. "Stevo's not someone who a batsman can use pace against to get off to a flyer. You're going to have to hit Stevo if you want to get boundaries. That's always a bit of a challenge when two batters are on nought to take a risk."

Even in the new batting age, relatively few are willing to take that risk - or at least seem to struggle to do so successfully - against Stevens or anyone else. Since 2012, the first over of the innings goes for an average of 5.92 across T20 cricket: comfortably the cheapest in the entire innings, even with the benefit of the fielding restrictions.

"Kent are fortunate that they can employ as a spare bowler someone who stands top of their Championship averages"

Using a bowler like Stevens in the opening over has another advantage. It is the one time in an innings when a bowling side can pre-plan to exploit a particular match-up, knowing that a certain batsman will be in and new to the crease, and they can bowl a type of bowler that he struggles against. "It's about 'this is probably the best match-up for this situation and at the same time we're pinching an over'," says Tom Moody, director of cricket for the Caribbean Premier League.

Stevens concedes runs in the opening over in line with the overall average. But his opening overs are still a significant boon for Northeast, because of the possibilities they open up later in the innings. When he bowls the opening over it means that Kent do not have to bowl all their main five bowlers out, and have flexibility in case one is struggling.

Using Stevens to open also saves quick bowlers to bowl at the end of the Powerplay, when batsmen are set and still have the benefits of the fielding restrictions.

"Those are the times when the batters are in and they're trying to get after you," Northeast says. "Very rarely after that first over do they keep on having sighters."

Since 2012, the fifth and sixth overs of the innings go for 7.93 and 7.96 runs an overs: the highest of the entire innings until the last five, and so they are a time when frontline bowlers are particularly essential.

Stevens this summer has shown the peculiar tactic at its best - but it does not always work out so well. Most obviously, there is the chance that the bowler, especially one not as wily as Stevens, could be smashed. Kent are fortunate that they can employ as a spare bowler someone who stands top of their Championship averages with 39 wickets at 18.51 and who has taken 398 first-class wickets in 21 years as a professional.

But even if the extra bowler avoids punishment, using this device for the first over still involves a fundamental trade-off: more options for the fielding side later in the innings, at the expense of less likelihood of taking a wicket at the start. Indeed, even Stevens has yet to take a wicket in an opening over this summer.

Does the tactic work? A few seasons ago, several counties slipped in an over of spin at the top of the innings. But it is still surprisingly rare - a non-frontline bowler only bowls the first over around 5% of the time, as per analysis from Joe Harris, of White Ball Analytics. His research shows that the average opening over delivered by a specialist bowler goes for 5.91 runs, and picks up 0.20 wickets.

When bowled by a part-timer (identified as one who bowls an average of two overs an innings or fewer), the first over concedes 7.42, for the loss of only 0.13 wickets. Most non-specialists are far less effective than Stevens in bowling those introductory balls.

The question is whether this reduced effectiveness in the opening over is worth it to enable increased flexibility. The answer will depend not just on the skill of the sixth bowler, but also upon the opening batsmen. A pair who begin relatively slowly are ideal to bowl a sixth bowler to; those who are particularly adept at attacking from the opening ball are more problematic. Tom Cooper's part-time offspin was outstanding opening in the last Big Bash League - until he encountered Brendon McCullum, and the first over went for 14.

Although the opening over of the innings remains the least expensive in T20, teams are becoming more aware of the need to target it. The run rate in the first over so far this year is 6.51 across all T20 cricket, compared to 5.96 last year. So for non-specialists, even bowling the first over is becoming no barrier to being attacked, especially as pinch-hitters - like Sunil Narine in the IPL, and Lewis Gregory and John Hastings in this year's Blast - are enjoying a revival in T20.

"As in all aspects of T20, perhaps the best answer is flexibility"

As well as the opposition batsmen, bowling teams must consider whether opening with a non-specialist would dilute one of their greatest strengths, by denying a strike bowler the chance to bowl to a pair of batsmen who haven't faced a ball.

As Moody says: "Regardless either of the idea of trying to pinch an over, or a recognition that it's a particularly good match-up, there are certain bowlers - Mitchell Starc or whoever - who are of a greater influence. They're the most likely bowlers to be taking a wicket early and that's your greatest value - wickets in the Powerplay."

As in all aspects of T20, perhaps the best answer is flexibility. If a non-frontline opening bowler is used in every game, batting teams can respond accordingly. But if used occasionally, with an ideal bowler in ideal conditions, it can prove extremely handy.

Moody notes the attractiveness of bowling an offspinner to Chris Gayle in the opening over; England did something similar when they deployed Joe Root, who used to open regularly for Yorkshire in the T20 Blast, to Gayle in the World T20 final last year. It was actually the second over, but Gayle had yet to face a ball: Root's first to him conceded a boundary, but his second was heaved into long-off's hands. And consider the record of Angelo Mathews, a bowler similar in style to Stevens. Opening in T20 cricket, he has taken 11 wickets at 11.72 in the opening over, at a startling economy rate of just 4.6.

There is no right answer about whether a team should bowl one of their weaker bowlers at the start of an innings. Like all T20 strategy, the decision is determined by a mixture of meticulous forward planning and instant in-game judgement. But any young T20 batsman who can bowl useful medium pace or spin would be well-advised to learn from Stevens: mastering the art of bowling the first over would boost their worth in the Blast and leagues around the world.