I havent made it to advanced yet, but my 3 partners fly it and we talk about snaps a LOT and I judge them on the advanced pattern almost everyday.
The snap has to be on the centerline and in the center of the downline of the 45 degree inverted dive, not 50, not 35, 45 degrees. Must see at least some break in the nose (down elevator) to start the snap but not so much as to change the heading of the downline. The break happens so quickly that if you dont know what you are looking for you probably wont see the nose break, but that break is what a big part of what stalls the plane to initaite the snap. The radii of the entry and the exit of the 45degree dive should be the same. If there is not a definite negative break in the nose to initiate the snap, its not a snap roll its an axial roll, and thats a zero. The speed of the snap isnt supposed to be downgradable but seems like the faster snaps get better scores from what I have seen at contest. Plus if the snap is too slow the break at the beginning will probably throw you off the original downline heading .. ie enter at 45 degrees and exit at 35 degrees.
Most common problem I see outside of the maneuver itself is letting the plane come in or go out just after the snap as the plane re enters inverted flight for the 2nd half of the downline. Got to be ready to get on the rudder the moment the snap stops to keep the plane on heading parallel to the runway.

Im sure I will get corrected but I think thats a pretty accurate summary of what I look for when I judge.

Mike