-dV or negative delta V, or -dV/dt in this context, stands for the voltage drop (decrease of voltage) NiMH batteries exhibit when they enter overcharge. When NiMH batteries are fully charged, they cannot absorb more energy and start to heat rapidly, the extra energy being dissipated as heat. The increased temperature causes decrease of internal resistance and the voltage decreases as a result. This is the so called negative dV or -dV. Sadly, most chargers use this method to terminate the charge of NiMH, thus overcharging the batteries, how much, depends on the value of the -dV the charger is capable of measuring before terminating the charge. Typical values are 10-15 mV for cheaper chargers.
The proper method of charging NiMH batteries is 0 dV, i.e. the voltage plateau that occurs just before the voltage drop, or just before the batteries enter overcharge. If implemented properly, this is the only method for charging NiMH that ensures full charging without overcharge and overheating at charging currents 0.5-1 C.