An illegal bowling action is one in which the bowler's 'elbow extension' exceeds 15 degrees while he is in his delivery stride. The ICC set the 15-degree limit for all bowlers in November 2004.
Elbow extension includes flexion (in this case, the closing of the elbow joint) and extension (the straightening of the elbow joint).
If the arm is bent at the onset of the delivery stride but remains rigid or does not flex or extend beyond the permissible 15 degrees during the duration of the stride, the action is not illegal. An action is only illegal if the arm flexes or extends beyond the permissible limit while in the delivery stride.
Once the match officials' report is received by the bowler's team management or home board, he must undergo testing on his action at an ICC-accredited facility within 21 days. At present, there are ICC-approved centres in Brisbane and Cardiff, and another one in Chennai is expected to be functional soon. The player is free to bowl until the results of the test are out.
The bowler is expected to replicate the action he uses during an international match and bowl at the same speed too, for the various deliveries being tested. His action is captured by multiple cameras and his movement is monitored using sensors placed on his body. The test, which is conducted by biomechanists and human movement experts, measures the degree of flexion and extension for every delivery and determines whether the action violates the prescribed 15-degree limit.
The player will be suspended from bowling in international cricket immediately and a report of the test will be sent to the player's home board. On receipt of the report, the board has the option of appealing the results to an ICC-appointed bowling review group (BRG) within 14 days. However, should the appeal fail, the BRG could impose a ban on the player for a period of time.
The player will have to undergo remedial work on his action. He can apply for retesting at any point of time, and if his remedied action passes the ICC's 15-degree rule, he will be allowed to resume bowling in international cricket.
If the player is suspended a second time for an illegal action within two years of the first instance, the second suspension - from bowling in international cricket - shall last for a minimum of one year. He will be allowed to apply for reassessment only at the completion of the one year.
If testing shows that the bowler's action is illegal only for a particular delivery, say the doosra, he will be banned from bowling just the doosra in international cricket until he corrects his action for this particular ball and has it passed as legal. If he is found to have bowled the doosra in an international game without having it reassessed first, he will be reported and suspended from bowling in internationals altogether, and the suspension shall be considered a second suspension in keeping with the terms mentioned in the previous question.