We totally get your confusion, and are confused about that point too:
1) On the Crest website, the instructions are vague and imprecise.
2) In the comments on that page, some comment posters state they have worn the strips for an hour first, then afterward applied the light.
3) We called the Crest helpline. That person said the light was applied to the strips for a part of the time while they are worn. But this person seemed unsure. They certainly didn't know the answer off the top of their head.
4) Since the Crest phone representative seemed confused, we decided to email Crest and get an answer that way. Unfortunately that response will take a few days. When we get an answer we will post it here.

With other whitening methods, the bleaching light is applied when the whitener is on the teeth.

Probably more important to state, we'd be of the opinion that the use of a light adds little if anything to the whitening process. It would be our guess/opinion that Crest's purpose in packaging a light with their strips is that it helps to sell more kits (is a gimmick), as opposed offering a scientifically proven technique that substantially adds to the whitening process.

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