So far, this thread has seemed to produce different descriptions of what is happening during an elevator induced snap roll, but not the reason why. I will try my best to explain the why and how it happens. The guy talking about separation of the air over a wing has got it right, sort of... The air separates starting rearward on the cord. When the critical angle of attack is achieved the separation has moved almost entirely forward toward the leading edge of the wing. This is an explanation of a stall but not a reason why the elevator can induce this.

First, a little fact that has to be understood.

A given airfoil (wing) will alway stall at the same angle relative to the airflow across it. This is called the "critical angle of attack". All wings have a 'CAA' and the angle varies from wing to wing depending on what purpose it was designed to achieve. Again, if you believe that a wing will stall when you achieve "IT'S" CAA then you can continue on... This has nothing to do with airspeed so it's hard to make this assumption without some aerodynamic theory background.

Remember that the airflow across a wing is relative to the wing and nothing else.

OK, let's continue. Let's say our test 'wing' has a CAA of 18 degrees. It's a relatively common, but high number actually, a trainer wing. While flying slow, as you pull up elevator and approach 18 degrees nose up a stall develops. That is because the air hitting the wing is near parallel to the ground and the wing is 18 degrees relative to it. Just prior to the stall you add full power. The airplane accelerates on an 18 degree up line. As it accelerates the air over the wing becomes more relative to the cord line. The angle of attack begins to approach 0 degrees even though you are 18 degrees relative to the ground. OK, if you understand this then the elevator induced stall (snap) will be easy to understand.

Get back into your hotrod model airplane. Lets say the Extra has a CAA of 15 degrees. You fly your model at a very high speed of say 90 knots. You have a huge overly authoritative elevator with lot's of throw. While flying straight and level, 90 knots, you crank in 45 degrees up elevator. What happens is this. The airplane nose rises very quickly, let's say to 60 degrees nose up. If you approached this flight angle slowly the Extra with it's big motor would crawl right up that 60 degree up line. The relative wind angle would remain near 0 degrees is why. But when you yank in this 60 degree nose high angle the inertia of the airplane makes it momentarily want to continue on the level flight path it was on parallel to the ground. With the wings 60 degrees nose up and the aircraft trying to push straight and level for a few split seconds you have "WAY" exceeded the CAA. A stall is the result because the law is that when you exceed CAA it will stall, regardless of orientation. It snaps because one wing always stalls before the other (unless purely built identical IE. mirrors, not to mention P factor) and because of the high airspeed the difference is exagerated. Thus a snap roll is the result.

There are two ways to reduce this tendency.
1. Reduce the speed in which the elevator is applied so the CAA is not exceeded. Gives time for the inertia to be overcome.
2. Reduce the wing-loading (aircraft weight) so there is less inertia to overcome. This allows the airplane to change directions faster keeping the relative airflow over the wing less than CAA.

I hope this helps explain "WHY" elevator induced snaps occur.