It is a heck of a ballsy move that very few people could get away with. He has a wall of 3 people abreast in front of him, Simon comes down to the red line to box him in, Hoy literally elbows him out of the way to create the hole in the wall that he needs, and just powers through it.
To make it more impressive is that this change in strategy was executed in 4 seconds:
4:36 he is thinking of going over the top of the riders
4:38 he has decided to go under or through the 3 riders
4:39 Simon blocks him in
4:40 He has elbowed Simon out of the way and has clear view of the finish line (and the power to seize it)