https://youtu.be/4T9UQ4FBVXI https://youtu.be/Bmjr4Q6je8I

Basically, you can’t bench with elbows straight out, because you’ll hurt your shoulders.

So you tuck your elbows a bit, but that means you’re fighting some leverage because the bar is no longer above your shoulders.

To fix this, you arch your back, which rolls your shoulders back and puts the bar back above your shoulders, but with your elbows still tucked.

One side of the arch, your shoulders, is pinned to the bench by the weight of the bar.

The other side of the arch, your butt, is not. So your arched back will straighten out under load, which rolls your shoulders forward, which moves the bar out of position.

So, to keep the arch solid, you use your legs to push your butt toward your shoulders, thus reinforcing the other side of the arch, keeping your shoulders rolled back.

So, leg drive is _not_ driving the bar, it’s driving your butt toward your shoulders, to keep the arch in your back solid, to keep your shoulders rolled back (and chest up) to keep the bar above your shoulders with your elbows tucked a bit, to keep from impinging a tendon.

You could bench with your butt off the bench and make a big arch, though this article explains you’re diminishing your returns by reducing range of motion.

https://startingstrength.com/training/keep-your-butt-on-the-...