What "undercling" means in rock climbing

In rock climbing, an undercling is a type of handhold which requires the climber to grasp and pull from underneath the hold, rather than from above it. Underclings are often used when climbing on overhanging or steep terrain, as they allow the climber to maintain contact with the rock surface and avoid swinging away from the wall. Climbers may also use underclings to stem or traverse across a feature, such as a corner or crack.