Quote (CMBurns @ 29 Jun 2017 20:01)
If your back is flat on the bench with no arc, mechanically speaking you'll be involving shoulders more than they need to be. It'll be harder to increase the weights that way and it's easier to get hurt.
To primarily involve your chest, you gotta push in front of you, slightly angled down (which makes absolutely no fucking sense on a bench that's perpendicular to gravity's pull). So you raise your chest, pinch your shoulder blades and pull your shoulders back to try and sink them into the bench. That also reduces the range of motion a little, gives you the right angle and drastically reduces the shoulder involvement.
tl;dr:
Touch chest.
Idiots who say don't touch chest are retarded.
It's implied in what CM said but elbows shouldn't be pointed way out either (pushing slightly angled down and forward typically prevents this). A lot of people bench with elbows up and out, as though you could draw a straight line across your shoulders and your elbows would be fairly in line with it. This is bad. Elbows should be down and tucked closer to your torso. It does 2 things when they're up and out, first it puts more strain on your shoulders (rotator cuff) and the second is it puts extra tension on the lateral part of the pec by stretching it which raises the potential for injury to that area as well