Things the bottom arm is there to do:
Keep balance behind the pole (this is why the one-arm drill, a very popular American excercise, is so ridiculous)
'Lat pull' (debatable)
Assist with the pull-turn (relatively unimportant, but true)
Things the bottom arm is NOT there to do:
Bend the pole (DUH)
Move the pole in any fashion (pretty self explanatory)
"Collapse" in after the swing (yes it does this, but it is not something to focus on in itself)
My biggest problem was collapsing the bottom arm on poles that I should of been capable of pressing out.
It was probably just a poor take-off and swing. Pressing out? It's pole vaulting, not pole bending or pressing.
If sliding boxes are bad for blocking and wide grips not good then surely just softer poles then!
!?
Or just use better means of training... And it's a biomechanical fact that a wide grip is bad for the plant, take-off, and swing, even the fly-away. (As KB just described!)
I suggest you try to think of the vault on a flexible pole in terms of vaulting on an infinite series of stiff poles. No, it is not a comfortable thought, but neither are irrational and imaginary numbers... and they exist.