We present a physical model for electronic switching in the cantilever-based nanoelectromechanical FETs, focusing on the steepness of its switching curve. We find that the subthreshold swing of the voltage transfer characteristic is governed by two separate considerations. The steepness of the curve is improved beyond the Boltzmann limit when several dipolar charges sitting on the relay move together and amplify the active torque. The steepness is also improved by electrostatic destabilization and pull-in forces that abruptly close the airgap between the tip of the cantilever and the drain, and exponentially enhance the tunnel current. For small sized relays, dipolar and short-range van der Waals sticking forces dominate, while for longer cantilevers the capacitive energy acquires a major role. The individual pull-in and pull-out phases demonstrate a remarkably low subthreshold swing driven by the capacitive forces, sharpened further by dipolar correlation. The sharp switching, however, comes at the expense of a strong hysteresis as the metastable and stable states interchange along the forward and reverse phases of the voltage scan.