This is how I won against an AI army twice as strong as mine. I attacked them at the entrance of a canyon, I put my 2 archers on each side of the canyon at high level and one unit blocking the way through in a spot that was facing only one of their units. I rained arrows on them and switched between the other units to have always the healthiest one blocking the way until I killed them all. I didn't even lost a unit.
I don't think that's exactly what he meant.
E.g. if you corner an army where they have no other spot to move to (e.g. against a canyon), their deployment zone will only be one spot and they can only deploy one unit. Its very much broken that it's possible to box the deployment zone in that much... I was on the receiving end of that once. Even though there would've been a path out for me, the deployment zone only gave me one spot. Very dumb.