Need some information on western collars?

Okay, so recently at rodeo's I've been seeing a lot of people attach their collar under the swells of the saddle instead of the sides. Why is this? My friend called them pulling collars. Also what is better for your horse a Y shaped collar, or the ones that have more body and curve? Which one frees up the shoulders more? My Quarter Horse gelding often has trouble with collars because if adjusted to high when he lowers his neck is becomes extremely tight, and if to low it will fall below his shoulders and restrict movement. He also has a small chest so I was wondering which type of collar would work better for him. We currently use a Y collar.