snap rolls
 

Can someone describe positive and negative snaps, stick positions and timing of unloading elevator?

RE: snap rolls
 

Positive snap uses up elevator - rudder and aileron in the same direction.
Negative snap uses down elevator - rudder and aileron in the opposite direction.

In either case, lead with the elevator. A split second later (after you see the nose pitch in the correct direction), add ailerons and rudder. The amount of throw required for each surface depends on the plane, throttle setting, speed, and direction (downline snaps easiest). Some flyers unload snaps after the plane stalls and begins autorotation by backing off on the elevator and rudder throw. This makes it easier to exit on a precise heading.