Heh... you would have to ask that! Ideally, and with a horse who understands what you mean, I bet it doesn't. I think because the snaffle is a contact bit, people usually don't ride with it slack like they would a curb, I never see slack reins on even young western horses in snaffle, they are usually riding on contact and sometimes with a martingale. They try to neck rein on contact, which gives a mixed message.
OK,:), I was just getting confused.
Becky