Some hold under the belly, some hold by the collar... it's because once your dog knows it's a game and a race, as soon as they hear "ready, ready, go" they go.. and when you are running with a team you have to wait your turn. It's just a restrain and in someways gets your dog pumped up more because he/she wants get outta there.

I hold Jersey by the legs usually, and hold them out from her body because it really helps to pump (you can see her tense up just before I release her) her up.