Bumper jar:

A device made up in the drill string that, when actuated, delivers a heavy downward blow to the string.

A bumper jar has a hollow body that moves upward when the drill string is picked up.

When the string is dropped quickly, the jar body produces a sharp downward blow on the tubing or pipe made up below the jar.

If downward blows can free a fish, a bumper jar can be very effective.