This sounds like the Bag & Tag execution, from the Gears of War Wiki:
This execution is performed after taking a downed enemy for some quick cover as a meat shield. This is done by tapping up on the D-pad when you have a downed enemy as a meatshield. This will make the player tag the meat shield with whatever grenade they happen to have in their inventory, then kick him/her towards the enemy causing them to sort of quickly stumble toward the enemy if there is an enemy you're aiming at within proximity. The idea of this execution is to take out a nearby enemy or to put him at a disadvantage at least if you had a smoke or ink grenade.
You may also want to read the article on Meat Shields, which has some interesting excerpts:
The Meat Shield can be "dropped" at any time by performing an execution move with the "X" button, in which the player's character will snap the neck of the Meat Shield and drop the corpse, or simply by switching weapons.
This doesn't seem like it would give you the "push" you're speaking of though.