In the most severe cases, the notch does in fact touch the zero voltage axis. This causes the biggest problems. During normal conditions, there are two zero crossings in each cycle. Some electronic equipment is designed to be triggered on the zero crossing or when the voltage is zero. This allows equipment to be activated without the surge currents or inrush currents that would be present if switched while voltage was present.
Some equipment uses the zero crossings for an internal timing signal.
Some digital clocks operate this way. When notches are present, particularly in three phase equipment, we can experience extra zero crossings. Instead of two zero crossings in each cycle of voltage, we can actually experience four notches. Think about it. That means the equipment will turn on twice as fast, run twice as fast or turn on at very wrong times resulting in damage.