A lawn dart is a large dart that one threw at a ring on the grass (hence the name, "lawn dart"). The safety nazis outlawed them because, if you were a real moron, you could kill yourself or someone else with the thing. The overall flight trajectory was sharply downward.
A "Lawn Dart bounce" is when the candidate comes out of the convention lower than when he started.
I didn't know lawn darts were outlawed. I'm one of the idiots that was injured by one. It isn't a good idea to stand just behind the other guy's ring barefooted. But hey, I was only about 8.