You stop the bike to the left of the clothes icon so as it rotates-- the edge touches the bike-- jump off and then back on the bike.
If the clothes change and you end up standing next to the bike, you were too close.
If you end up on the bike and your clothes don't change, you were too far away.
Here is a (bad) pic of it.
If you get it, don't use your brakes to powerslide as this also will select your weapons-- this can make you come unfused. He will definitely unfuse if you fire a weapon.
Good places for fused biker jumps---anywhere

-- if you want more rotation, pop a wheelie and turn a little as you hit something--my favorite is the fountain on Prawn island.
Hitting ranchers or any vehicle going fast works good too.
Fusing is also a prerequisite for superjumps