How does it keep the ladder from slipping out? Looks like the whole thing would move under the ladder.

We put non-slip rubber pads on the bottom of the stabilizer and the platform to keep the Ladder-Aide from moving, and 1/4" lips on the platform keep the ladder's feet securely in position.

One of our tests involved loading weight onto a ladder on the Ladder-Aide, then applying 50 lbs of lateral pressure to the Ladder-Aide, pulling it from the side. A "pass" would be less than 1 inch of movement. It doesn't budge at all. Watch the test being performed in the video below.