The present investigation shows that a triad of three vortices is behind the quasi-periodic self-sustained low-frequency flow oscillation phenomenon. Large-eddy simulations are carried out at Rec 5 × 104, M∞ 0.4, and sixteen angles of attack at near-stall conditions. It is shown that the best description of the underlying mechanism is that of a button whirligig. A globally oscillating flow whirls around the airfoil in the clockwise direction and stores energy in the triad of vortices. When the oscillating flow loses momentum and comes to an equilibrium, the energy stored in the triad of vortices reaches a threshold. Thus, the triad of vortices reverses the direction of rotation of the oscillating flow, gives it a pulse of energy to whirl in the anticlockwise direction, and starts a new disequilibrium. When the oscillating flow rotates in the clockwise direction (in the streamwise direction on the suction surface of the airfoil), it adds momentum to the boundary layer and helps it to remain attached, and vice versa. Consequently, the instantaneous flowfield switches between an attached phase and a separated phase in a periodic manner with some disturbed cycles.