It's down to when the weight of the bike shifts between the front and the rear, usually in corners when you come off the brakes and onto the throttle.
If there is no rebound damping, the bike will "pogo", shifting the weight between front and rear in the corner.
Obviously this is an extreme case, but not enough rebound means the weight shift from front (on the brakes) to rear (on the gas) as you come out of the corner happens too quickly and varies the front grip too much so you may lose the front.
Other effects are that it may just unsettle the bike or not feel safe on mid corner or during brake/throttle transition.
Have a look here for more tips:
http://www.v-twins.cc/info/AdjSusp.htm
antonye
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