How does hand position in a dumbbell flat bench affect which muscles are activated? The positions I know are palms facing each other and the traditional palms facing down. On rare occasions I see people do it with palms facing upward.
When doing dumbell presses, I alternate palms facing in and palms facing out. You can alternate on a rep by rep basis, or a set by set basis, or a workout by workout basis, or even stack by stack basis. Palms facing in or out gives different stretches at the bottom, works your forearms differently, and depending on angles, can hit your triceps differently. Experiment with them and see. By alternating, you hit every single muscle fiber. Reverse grip bench press with palms facing up puts more stress on your wrists and focuses the triceps. Normal grip with palms facing up focuses on the triceps. The latter two are not used that much traditionally.
Also, wide grip bench puts emphasis on outer chest and shoulders, while close grip bench puts emphasis on inner chest and triceps.
Here's the only valuable article I could find: http://www.fitflex.com/hand-position.html
For overhead presses the hand position seems to change where the focus on the delts are most felt......