However, since both reeds participate in producing the bent note, the natural note of the lower pitch reed in the hole actually raises while the higher pitch reed lowers in pitch. For example, for a draw bend the pitch of the draw reed gets lower while the pitch of the blow reed gets higher. The note in a hole can be bent down to about a semitone higher than the lower pitched reed in the cell. This is the best I've been able to determine on the physics of how bends work on a diatonic harp. Each reed has a range of vibration frequencies to which it will respond.
Resonance adds energy to a frequency because of reinforcing wave forms.
The range of frequencies to which a reed will respond overlaps for both reeds in a cell. The reed will respond to the driving compression wave frequency with the highest energy. Both reeds in a 2-reed cell will respond to the same driving frequency because that driving frequency falls in the range of frequencies to with each reed will respond, that is, where the response frequencies overlap for the two reeds. So the note that sounds depends on the resonant frequency of the airway tract, subject to the mechanical response characteristics of the reed pair. Thus, bends are induced by changing resonance characteristics in the vocal tract, and the reed vibration rate is coupled to the playing tract.