Andrew McGlashan is a deputy editor at ESPNcricinfo
Between lunch and tea on the third day in Karachi, Australia's bowlers claimed six wickets - double the tally they had managed in the series until that point. There were only 20 overs in the session but more happened then than the preceding 22 combined. Barring a miracle, it was likely the moment where Australia earned just their fourth Test win in Pakistan.
There had been early signs that the scoreboard pressure applied by Australia's 556 would tell on Pakistan: an attempt at a non-existent single had brought the first wicket, then Imam-ul-Haq's reckless strike against Nathan Lyon had given an Australia bowler a scalp for the first time in more than 600 deliveries.
But the moment the day, the match and possibly the series changed came in the 21st over when Pat Cummins thundered a full delivery into Babar Azam's pad. Squeezing it off the face of the bat saved Pakistan's captain, but the signs were there: reverse swing was in town.
It was not the first time the ball had tailed in the match - Shaheen Shah Afridi was probing with it through Australia's long innings and Faheem Ashraf also threatened - but for the first time it would have a telling impact. Cummins, who had an excellent day as captain, sensed the moment. Lyon had just claimed the wicket of Imam but he was whipped out of the attack in favour of Mitchell Starc. There would only be one more over of spin in the session.
Australia had been planning for this moment since before the tour during their pre-series camp in Melbourne where they put considerable time into the bowling of reverse swing. "A lot of time the ball reverse swings so we didn't really experience it this summer with grassy wickets, short games, whereas over there it can be a real weapon so trying to upskill that," Cummins had said before flying out to Pakistan. "We haven't bowled a lot of reverse swing in the last year or so but it's a huge factor going into the subcontinent."
It had not really transpired in Rawalpindi on a lifeless surface and a slightly more verdant outfield, whereas at the National Stadium there is a wide square of barren pitches ready-made to scuff up the ball. Before the match, the likely role of reverse swing was front and centre in Australia's selection with the retention of Starc (the current Allan Border Medalist as Australia's men's player of the year) ahead of Josh Hazlewood. It's not that the latter cannot be effective with the reverse, but Starc's left-arm angle and few extra kph brings an added dynamic and both were on show in the over that truly began Australia's surge.
From round the wicket - an angle that allows him to push the ball into the right-handers then have the movement either slant it back further or take it away - he lured Azhar Ali into poking at a full delivery outside off stump which sent a catch whistling to second slip where Cameron Green snaffled it with deceptive ease given how close he was standing.
Next ball, to the left-handed Fawad Alam, Starc was back over the wicket and produced a trademark full delivery which curled into Alam's pads in front of the stumps. On the eighth day of the series it was Fawad's first opportunity to bat (he has not bowled and also dropped a catch in Rawalpindi, although held on to remove David Warner in the final session) and it was over before there could be the usual freeze frames and analysis of his unorthodox stance.
On a hat-trick, Starc saved his best for Mohammad Rizwan, and it was too good. A length delivery that jagged away off the surface from round the wicket to beat the edge. It was not a million miles away from matching the famous delivery he sent down to James Vince at the WACA during the 2017-18 Ashes. The over was on a par with the one he bowled against England on that heady second evening at the MCG just a few months ago when Australia's quicks produced one of the more unplayable passages in recent memory.
Pat Cummins celebrates the wicket of Mohammad Rizwan•AFP/Getty Images
There was no reprieve, though, for Rizwan. After the drinks break Starc was replaced by Cummins who gave Rizwan a torrid time. He was dropped at slip by Steven Smith who went for a catch that was probably Alex Carey's for the taking. Next ball he padded up to a delivery which jagged back and was given lbw when it was nowhere in the vicinity of the stumps - DRS providing the perfect example of why it was first introduced.
The relief was momentary, however. Just two balls, in fact, until the start of Cummins' next over when he found Rizwan's outside edge with another perfect delivery in the channel. This time the edge was finer and there was no doubt where it was heading as it nestled in Carey's gloves.
Through all this you had Green showing, again, the value he will bring to this Australia side as he enabled Cummins to keep going with pace from both ends and removed Faheem Ashraf for good measure during a six-over spell. Such was the impact Australia were having with the quicks, that when Green had to briefly leave the field after a blow on the hand, Marnus Labuschagne had an over of medium pace.
There was time for Starc to strike again before tea, although his third wicket owed much to the excellent hearing of Carey who was convinced Sajid Khan had got a thin edge which was proved correct. Throughout this, Starc maintained an average pace over 140kph, the only bowler to achieve that in the match. When the ball did move, either off the pitch, in the air, or both, that extra speed hurried all the batters.
The final session of the day was a touch anticlimactic after all that as the lower order was wrapped up by another direct hit run out from Labuschagne - do not discount the value of Australia's fielding - and the first two wickets of Mitchell Swepson's career. This was the perfect day. Australia's bowlers will need to do it again tomorrow, but on the latest evidence they have all the tools available. After reaching until almost the mid-point of the series, this may just have been the moment it took the decisive (reverse) swing.