Most bicycles have a freewheel mechanism. When pedaling forward they engage inner teeth with things called pawls. When the rider stops pedaling the bicycle will coast. This is because the teeth that the pawls engage with are ramped on one side and push the pawls up out of the way. This is the clicking sound you hear when coasting. When you pedal backward you are really not doing anything different than coasting, at least at the freewheel mechanism, You just speed up the motion by pedaling in reverse because everything is moving in the same direction as coasting. But as soon as you start pedaling, the pawls catch the non-ramped side of the inner teeth and engage the whole hub (inner part of a bicycle wheel) and propel the bicycle