Jumping onto an already rolling bike with your shoes waiting in the pedals. Executed cleanly it saves 15–30 seconds — the flying dismount, by contrast, only 2–3.
Over an Olympic distance lasting barely three hours, 15 to 30 seconds is small but real — and it costs no watts and no equipment, only practice. That's exactly the catch: the flying mount is the one transition technique that, when it goes wrong, costs more than it could ever save. A botched attempt in a crowded transition area means a crash or a collision, not a few lost seconds. So rehearse it dozens of times slowly on an empty car park before it ever appears in a race, and build the base routine first: a fixed spot for helmet and shoes, nutrition mounted at bike check-in. That saves more time than any trick. Two things that get missed: at 2 to 3 seconds the flying dismount is a poor trade against the risk of a fall or a penalty. And some organisers penalise mounting before the line regardless of how good it looked — read the athlete handbook.