A kneebar is a body position where the climber jams the knee or thigh between two opposing surfaces — typically a hold above the knee and a wall or feature below — creating enough opposition to support body weight without arm strength. When done well, a kneebar can be a true no-hands rest mid-route, even on overhanging terrain.
Kneebars appear in roofs, steep dihedrals, and any hold geometry that creates a "tunnel" the leg can wedge into. The climber walks the leg into position, twists the hip slightly, and locks the knee until the position feels solid. With both hands free, the climber can chalk up, shake out, plan the next sequence, or simply rest pumped forearms.
Kneebar effectiveness depends on the rock features (or hold size) and on the climber having a "kneepad" — a thin foam pad strapped over the kneecap to prevent the joint from sliding off the hold under load. Most modern hard sport climbers carry kneepads as standard kit. Finding a hidden kneebar can turn a boulder problem into a moderate sport route.