At what event would you find a quintain ?

At a Jousting competition
It is a counter balanced rotating target hit by a mounted horseman with a lance

It's my guess that a hit in the center of ther target swings the weighted sack around faster, and it misses the oncoming rider, going in front of him, while a glancing blow would cause the sack to turn a bit slower, and catch the oncoming rider in the back of the head.