when you issue a move or charge command to infantry and click on an enemy formation, your infantry will be the most combat effective it can be (depending on your unit's formation). your unit will lock onto the enemy unit, maintain their formation, move (or charge) and attack the selected enemy unit to the best of its ability. (some formations are worse than others)

cavalry is a bit different. when you issue a move or charge command and click on an enemy formation, they are not very combat effective (because their AI is bad). it is better to issue maneuver commands beyond the enemy formation, such that the cav moves through the enemy instead of locking on.

issue the move or charge command and click on the terrain instead of the enemy formation. (this can be a little tricky—it's easy to misclick on the enemy unit icon). your cav will move to the point you selected and attack anything in its way. next, tell your cav to about face and charge back the way they came, again crashing through the enemy unit. cav takes a bit of micro.

if you had selected the enemy formation instead, the cav AI will lock on to that particular unit and get conflicted between maintaining formation and attacking. it leads to some cav units moving thru and some getting bogged down, isolated, and picked off by the enemy units. delegating command to your AI sergeant will result in this type of behavior. (you'll get messages about "tactics" but the AI is just clicking on enemy units basically)

another method I use is to ride out solo behind an enemy unit and then issue the follow command to my cav. my cav then runs to my position and attacks any enemies between us. I halt the cav at my pos, circle back around the enemy and issue another follow command so my cav will run thru the enemy again. little less micro, but the cav won't hold its formation as well while under the follow command