The rate at which electrical current changes in the coil over time. Since the magnetic field is directly created by the current flowing through the coil, a faster current change produces a faster magnetic field change (higher dB/dt). Increasing the drive voltage or reducing coil inductance both increase dI/dt, resulting in sharper pulses with stronger tissue stimulation.
A magnetic field that changes in time induces an electric field in the tissue it passes through. This is Faraday's law, and it is why the speed of the field change (the slew rate) matters more than the field's peak strength: a faster change induces a stronger electric field, which is what stimulates the body.
