Feet-up bench press

Lie on a flat bench and grip the barbell with your normal bench press grip. Lift your legs off the floor and hold them up in the air.
Unrack the weight and hold it above your chest with your arms fully extended. Brace your core intensely to keep your body from swaying.
Lower the barbell with control to your mid-chest, keeping your elbows tucked at a consistent angle.
Without using any momentum, press the bar powerfully back up until your arms are locked out.

Tips

With your feet off the floor, your lower back loses its usual base of support, so press your upper back firmly into the bench and keep your torso still throughout each rep.
Hold your legs at a height that lets you keep them steady without letting your hips or lower back shift as you press.
Use a weight that allows you to press smoothly from the bottom without jerking or bouncing, since you cannot drive through the floor with your legs to assist the lift.

Letting the hips and lower back shift or rock during the press, which means the torso is no longer stable on the bench.
Using a weight that requires a heave or jerk to get the bar moving from the chest, rather than pressing with controlled force from the start of the rep.