Usually, there was a breath between casts. Now, there was only light.

WoW has a "spell queue" system (usually 400ms) to help with latency. A "No GCD" hack expands this window or bypasses the queue check, sending multiple CMSG_CAST_SPELL packets in a single burst.

In a true "No GCD" environment, the internal server-side check that prevents you from casting multiple spells in rapid succession is disabled.