If you pause this video right at 0:02 you’ll see a pretty glaring difference between Bolt’s mechanics and those of Gay next to him:
Bolt drags his left foot, while Gay has a more circular turnover, keeping his foot high.
Asafa Powell does the same thing:
It seems like some kind of device to encourage greater extension with the left leg right out of the blocks. I imagine it sets you up to have longer strides throughout the accel phase. What do you guys think about this? I’ve been working particularly on my start mechanics lately, and I’m tempted to try this to improve extension during accel.