> it must perform it's normal autoregressive decoding to know what is the correct token in order to have something to compare with

Correct except for the word "autoregressive". When you have to verify a sequence of tokens (which were autoregressively generated by the cheap model), you can do each token in parallel. This amortizes the cost of loading the weights from vram to the processors (the primary cost in LLM serving) across those tokens. Cost here is wall clock time, as well as power.

The autoregressive decoding that generates this batch of tokens is delegated to the cheaper model where the cost of loading the weights is lower and so not amortizing it is fine.

Verification means, how close is each token in this sequence to the one I would have output. You keep the longest prefix that is close enough for your liking.