Remember that some planes just dont want to snap, specially the ones with constant chord (broad wingtips), because in a snap only one wing stalls and that planes does not lik stalling wingtips

Posittive snap: add up elevator, ailerons and rudder to the same side
Negative snap: Down elevator, ailerons and rudder to the opposite side

Often you will want to do neg snap from flying inverted and possitive if you are upright