When cavalry/mounted units occupying an open ground hex but are engaged in hex-to-hex contact with a unit occupying a river,wood, broken ground, or built up area hex they fight as normal but lose any impact bonus.
A good example would be a unit of 4 heavy cavalry stands,with a combat factor of 5 and a +4 impact bonus, would fight with a value of 9 and not 13 as the impact bonus cannot be used.
Once the same heavy cavalry unit follows up or moves into a wood, river, broken ground hex etc. it would fight 'as if disrupted' in that it cannot count its impact or number of stands, only its combat factor of 5.
When occupying one of these aforementioned terrain hexes, the heavy cavalry can become disrupted in the normal way as a result of shooting or hand-to-hand combat and receive a red counter. This disruption can only be removed when the unit is no longer in hex-to-hex contact with an enemy unit at the end of its sides movement phase.
If the cavalry unit leaves the terrain hex and enters an open ground hex, either through normal movement or following-up after combat, then the unit will fight as normal; counting stands and any applicable impact bonus.