The clamp works but it is unnecessary
"Find a hard surface with a good, aquare edge, like a table or bench top. Hold the mag in your hand, floorplate out and the rounded end up. Place only the edge of the floorplate on the edge of the table. Push in on the pin with a small punch or armorer's tool far enough to clear the hole in the floorplate. While pushing down and forward with your tool still inserted in the hole, force the floorplate to slide over the locking pips and off the bottom of the mag. Once you succeed the first time you won't have any further problems."
Don't do it on a nice table though... a cutting board, floor, or workbench.