Historians such as John Keegan have shown that when correctly prepared against (such as by improvising fortifications) and, especially, by standing firm in face of the onslaught, cavalry charges often failed against infantry, with horses refusing to gallop into the dense mass of enemies, or the charging unit itself ...

How did cavalry charges actually work?

Usually cavalry charge fleeing troops or hit the flanks/rear of the enemy formations. In these situations if the enemy does not run, it's simply a matter of beating them in a melee or retreating to attack again.

Why were heavy cavalry so effective?

For much of history, humans have used some form of cavalry for war and, as a result, cavalry tactics have evolved over time. Tactically, the main advantages of cavalry over infantry troops were greater mobility, a larger impact, and a higher position.