Actually derived from an Ancient Roman training device for The Sword, The 12th Century Quintain was an important training tool for would be and fully commissioned Knights. Made of wood, the typical Quintain was 5 to 6-feet tall. A pole which was secured to a rotating pivot (which spun around freely, 360-degrees). An extension Arm (another wooden pole), was attached to the pole, which featured a shield or related circular target. Opposite this, was a series of counterweights on a smaller extention arm. This could be rocks or more commonly, sand.
In theory, a Knight training would strike the shield/target, which caused it to spin - according to applied force and return momentum. The harder the "hit", the faster the returning "spin". This caused The Knight to take evasive action to either counter the strike or avoid it altogether. Some Quintains featured actual attached weapons, to increase realism (and scars!)
Quintains were also used in training a Knight first he all important Joust. Using a smaller target, a Knight was taught to strike a smaller central massed target at a full gallop and charge. Like Sword training, and inattentive Knight could be knocked off his horse (or unhorsed).