This has probably been suggested dozens of times before, but here goes:
Basically, this is an event that will fire a relay back to whichever brick most recently sent it one. For example:
Events for Brick 1:- onActivate - [Brick 2] - [fireRelay]
[0] onRelay -
[Self] - [playSound] [breakBrick]
Events for Brick 2:- onRelay - [Self] - [fireRelayBack]
[0] onRelay -
[Self] - [fakeKillBrick] [0 0 5] [10]
What this would do: Brick 1 fires a relay to Brick 2 when clicked, which causes Brick 2 to fake kill and send a relay back to the most recent relay sender - in this case Brick 1. There are several applications for something like this, with the most notable being the ability to handle user input when you do not know what that user will do, and handling it without the use of VCE conditionals or extra named bricks.
Feedback and suggestions are appreciated.