As a set of contacts open while current is flowing, like with a contactor or disconnect, the current forms an arc across the gap until the air gap distance increases to where the dielectric is high enough to extinguish it. In an AC circuit that happens fairly quickly because the polarity is crossing zero twice per second to help out. With DC that doesn't happen. Although technically the PWM pulses coming from the VFD are DC, they too are going to be zero in between pulses, but that happens so fast that the DC arc is still there when the next pulse fires. But in the mean time because the pulses are turning on and off feeding an inductor (the motor), they make the air gap act like a capacitor, pumping up the voltage across the gap as it widens. That voltage is rising very fast and can quickly exceed the dv/dt rating of the transistors themselves, causing them to break down at their junction layers. Translate: fail.
Most modern IIGBT drives now have protection circuits on the outputs (flyback diodes, etc.) that will suppress this capacitive action. So opening once in a great while, like in a emergency, is unlikely to cause damage any more, however the potential damage is also incremental. So if the mfrs said that it's not immediately fatal to open a contact down stream, someone would immediately abuse the concept and blow up some drives, then say "Well you didn't tell me not to!" or "Define once in a great while." So instead, they leave the admonition in saying "Don't do it" as the default position. In addition, you don't know for SURE whether your particular VFD has or doesn't have that protection; some older and / or cheaper ones don't, so it's best not to press your luck.
