I can keep my legs on the floor and not use them to drive the weight,
To tell you the truth, I haven't benched with feet on the bench since I first started weights,
If you can still arch your back and keep stable, I see no reason not to do it. (using a moderate weight that is).
If I could try to explain how I truly isolate a muscle, I would tell you to forget everything you know about lifting,
grab a moderate/light weight
flex the muscle you want to isolate
while holding the flex, perform the movement without using any other muscle.
flex as hard as possible at the top of the movement, and flex on the way down.
no matter how much weight you have in your hot little hands, you should be in pain after 10+ reps.
otherwise flex harder..
that is how I isolate a muscle