Basically, a hot seat is generally someone who rides very actively with their seat. This means that you move and push a lot with the way you sit in the saddle. This can be a good or bad thing depending on what horse you are on. For instance, hot horses generally need a rider with a quiet seat and quiet hands to counteract their already hot temperament. If someone with a hot seat gets on a hot or very forward horse then the results may not be too good. However, if you have a very lazy horse that really needs a lot to get going, a hot seat may be slightly beneficial as it's another way to wake your horse up. Ideally, a reader should be able to control and adjust their seat and the way they sit on/move with a horse depending on the horse that they are riding. Note that a quiet seat is the opposite of a hot seat- someone who generally sits in a way that does not affect their horse; their seat is so gentle it rarely interferes with the horse.