What is the button Heero pushed when he and Relena first met in the beach?
According to the wikia the suit had a built in suicide bomb but it was damaged by the salt water. I would guess that this is to prevent the pilots from being captured. This would be similar to fighter pilots during War time using cyanid pills to keep from being interrogated for delicate information.  


  Washing up on the beach, Heero's unconscious body was found by Relena and, in an attempt to kill himself, pounded his chest detonator triggering the flight suit's built-in suicide bomb. However, the saltwater damaged the explosive which caused only his detonator to fire, knocking Heero over.