TLDR: they precisely timed when to cut the power to the main chip that does the Wacom pen tablet processing following requesting a block of bytes from the USB descriptors field, which results in the CPU failing to execute some function that probably prevents you from reading beyond say 32 bytes. By doing that they managed to read back the entire rom of the chip.. The timing and shape of the power glitch was found through a ton of brute force / trial and error and
obvious intelligence on the subject..
Side note: Wow.. I'm an EE and that was pretty amazing stuff.. Was not aware of this technique.. Tour de force I think is the phrase I would attach to this video..