Single-DB Chest Press: Creating an imbalance forces your abs to contract as you work.
Try this you guys!! t's AWESOME! You might want to start with a light DB to first feel the movement.
With a DB in one hand, put the leg on the opposite side of the working arm up on the bench. ALSO, place the hand on the same side as the leg up on the bench, on your stomach. Don't just let it hang. You should notice an immediate firing of the core stabilizing muscles that are fighting to hold you in this position.
Now you are ready to lower the DB slowly and then push the DB back to starting position. Once you get the hang of it, challenge yourself on the weight.
I did 12-15 reps 3x for each side. I am holding a 25lb DB. And you just worked your abs and didn't even know it.