There is a push into the rail. That is the first foul. The rail depresses and the balls are no longer frozen. The cue slides over on the cue's tip then comes off the rail.
There can also be a bouble hit caused by the rail's collapse. This would be the cue hitting the cueball as it releases from the push.
Definate foul.
Just see exactly what you can get away with. You may be fore warned shafts may split and ferrels crack. You also may just seperate the rail rubber. The object ball hides alot.
Can you can shoot that rock almost straight into with a clean hit when it is froze against the rail?
Don't break anything important!