Twin Cam engines fire both plugs at the same time but only one plug is in a position to fire a fuel charge and make power with it as opposed to a single fire system where each plug is fired independently of the other at the proper time in the compression stroke.

Single fire systems use one coil to fire both plugs as compared to older engines that had two coils and each coil would fire one spark plug.

When the wasted spark in the single fire system is on the rear cylinder it's just at the start of the intake stroke. The intake valve is wide open and if you ignited on that spark you'd get a backfire through the carb. Also the piston on the rear cylinder is descending so you would not get a kick "back" but a kick "forward".

When the wasted spark is on the front cylinder, it's during the exhaust stroke. Again... a valve is wide open but in this case it's the exhaust. You'd get a backfire out the exhaust if the spark ignited anything.

http://i149.photobucket.com/albums/s45/bills985/Wastedspark.jpg