Once an opponent has been thrown, it is usually vital to pin them down in some way to prevent them getting back up and attacking again. Often the best way to do this is to twist a joint and use moderate pressure to hold them down. Locks such as these are very effective as they usually work regardless of how muscular the opponent is.
Particular locks are easiest to perform after certain throws, but most can be mixed and matched with any throw depending on how the opponent has landed. Locks are often used on wrists, elbows and shoulders, but others target knees, ankles and necks.
When fighting on the ground, a variation of a lock called a holddown is used. This pins the opponent down often without much pain but wears them down by restricting breathing and/or movement.