Actually no,
The corkscrew can be as big as it needs to be. To visualize a perfect snap I try to imagine a string across the sky, plane is attatched at the cg to the string. Initiate snap with elevator input to break the nose, but dont "bend" the string by climbing or diving out of that line. Get out of the elevator and almost simultaneously add rudder/aileron a spilt second later to snap the plane. The plane should enter and exit the snap without "bending the string" but the nose and tail can revolve around the string as big as they want. Too small and it looks like an aileron roll. That applies regardless of the line you are drawing, could be level, straight up, straight down or on a 45degree line or whatever. Could even be done in a loop at the top or bottom(avalanche). Of course in reality the plane is going to corkscrew somewhat around the line the plane is traveling down, but the goal is to not do so.

I have never competed in IMAC but I know that in pattern if the conical rotation is so small that it cant be seen the maneuver is going to be scored a zero. Things need to be right to get a plane to snap cleanly. Funfly planes dont usually snap real well so they are going to look almost like a barrel roll when you snap them. Pattern planes will snap so well that if you dont input the elevator to "break" the nose an instant before inputting rudder and aileron it will look like an aileron roll. Extras, Edges, Giles will snap real easily in my experience, something to do with the wing design and short tail moment I think.

Here it is straight out of the AMA rulebook, to see this entire section of the rulebook click here
[link=http://www.modelaircraft.org/compreg.asp]Download "RC Aerobatics" Section[/link]

• Snaps—A Snap roll is a simultaneous, rapid
autorotation in the pitch, yaw and roll axes of flight
in a stalled wing attitude. The following criteria
apply:
1. Since the maneuver is defined as a stall maneu-ver
(initiated by a rapid stall of the wing induced
by a change in pitch attitude), the nose of the
fuselage should show a definite break from the
flight path in the direction of the snap (positive
or negative) while the track closely maintains
the flight path. The track visualized as the path
of the Center of Gravity (CG) should ideally fol-low
the geometric flight path of the maneuver
while the nose and tail autorotate through oppo-site
helical arcs around the flight path. A snap
that does not show a break and stall to initiate
the snap, but does enter a stalled attitude during
the maneuver is severely downgraded.
2. If the stall does not occur and the model bar-rel-
rolls around, the maneuver is zeroed. A bar-rel
roll can be identified when the CG, the nose
and tail scribe the same helical path through the
required rotation of the maneuver.
3. Snap rolls have the same judging criteria as
axial rolls as far as start and stop of rotation,
constant flight path through the maneuver, and
rate are concerned.
4. Snap rolls may be either positive or negative.
5. If the model returns to an unstalled condition
during the maneuver, such that the autorotation
is not visible and the model rolls to complete the
maneuver, it would be severely downgraded.
6. Airspeed is not criteria which should be used
to judge this maneuver. As the wing of the
model is stalled during this maneuver a signifi-cant
decrease in speed may occur and is not a
cause for downgrade.