For an experienced horseman it's not very hard to do...while conversely it's really hard to prove. It's much the same as the reason why a horse that is on the lead and starts to drift late in the stretch almost never drifts out of the path of a horse coming from the rear...and instead drifts into its path...as in hanging to the inside if the closing horse is coming up the pylons and hanging up the track if the closer is coming at it to the outside.
I'm a complete bozo in the bike, I sit like a 125kg bag of Chaff, and even with my modest abilities I can make one drift about if I wanted to & you'd be pretty hard pressed to see it.
http://www.grannyreilly.com.au/Grann...haff%20Bag.jpg