If you are pushing with your back leg, you are apparently moving forward. If you are reading the pitch while moving forward, you have already committed your swing to a certain area that must be adjusted on the 'fly'. This leads to less than good contact, if any at all, most often when large adjustment is called for.
Creating stretch and potential energy at this point is too late in the process since the bat head is already committed by the forward movement.
It would be similar to standing upright in the stance and then dipping your hips only to raise them again.
A 'downhill' swing is far more efficient.