Hi Lero, the rules apply to the unit with the shock cavalry rule not the unit it is fighting against.
So, for example, I have a unit of mounted Norman knights that are shock cavalry. They are fighting against some Vikings. On turn two, the Norman knights charge a unit of open order Hirdmen. The knights win the combat by a factor of 1 but as the Hirdmen are the first formed unit the knights have fought against the combat result is read one line lower, so the Hirdmen get pushed back.
Later in the game, lets say turn 5, the same unit of knights charge against a unit of open order Bondi. Again, the Knights win the combat by 1. However, as this is not the first formed unit they have charged or countercharged they do not benefit from reading the result one line lower and so the combat resolution remains at 1.
Hope this helps. Cheers W.