A normal diatonic harmonica has two reeds for each slot, one blow reed and one draw reed.
When playing a (draw) bent note these reeds work together, the draw reed vibrating at a pitch lower than the one it is tuned to and the blow reed vibrating at a pitch higher than the one it is tuned to.
When playing an overblow only the draw reed sounds while the blow reed is choked.
Playing the overblow can be seen as to consist of two separate parts:
1. Getting the blow reed to stop sounding.
2. Getting the draw reed to sound.
Though the objective in overblowing is to get the draw reed to sound while blowing into the slot, choking the blow reed is the important first step to achieving this.