I know that hitting or bumping the heart is a bad thing; it can throw off the heart's rhythm, sending it into Atrial (bad) or Ventricular(REALLY bad) Fibrillation. There's also the problem of a bruised heart, which opens up a whole can of worms.
It's part of the kernel of truth behind the mythical "death punch": if a trained boxer or martial artist punches you really, really hard in the ribs in exactly the right place it can cause either or both of these. This is kind of the reason we have ribs, in fact- they're basically a roll cage for the heart, lungs, and diaphragm.
On the upside, it's also one of the few times where "hit it again" actually can work sometimes. It's part of the reason that you do chest compressions in CPR, and you'll sometimes see people who probably should have a pacemaker hit themselves in the chest when they experience angina- you can sometimes bump the heart back into rhythm.