Say you are riding at about 50 MPH and you engine brake down to about 30. All of a sudden some punk kid in a riced out Geo with a fart pipe blows past you waving. You have to save face so you open up the throttle and notice a "kick" that feels like someine hit your rear tire with a mallet. That kick that you feel is driveline lash.
It's mainly caused by loose chains that take a split second to come under tention (It can also be related to your gear box, but that is far less likely). If you have bad driveline lash, the first thing you should do is examine your chain and sprockets. Make sure the chain and sprockets satisfy the tolerances specified in your shop manual. If they aren't, get yourself some new ones. Also, replace your chain and sprockets if your chain tensioners are all the way out.
A little bit of driveline lash is inherent in every chain driven motorcycle because the the chain cannot be perfectly tight since it needs a little slack to allow the rear suspension to travel.
I prefer to step lightly and carry a BIG stick...