The rationale is... if you raise your grip the pole will be soften and bend more.. thus lowering your center of mass more... making a shorter cpole... which helps the pole/vaulter rotate more...
Another way I was thinking about it was average cpole length. Can the average cpole length of a pole during a vault ever be decreased by raising the grip?
For the record, I would never tell a vaulter to ever do this....
