Could someone explain why we have to really "use the floor"? Like brushing your foot pressing onto the floor.... How/why does this help? I forgot. And how much is too much? Thanks

Because if you push hard against the floor, your leg & foot muscles get stronger. Just like doing pushups -- your arms get stronger.!

If you "use the floor" in a battement tendu, the muscles will automatically engage and the leg will remain "stretched" or straight. Don't push so hard that you lose the lift in the supporting hip...