Timing of the two waves
A study needs a clean "before". So the pre-wave must finish before any reader sees your campaign, and the post-wave must wait until they have.
Brand Lift studies start at Elite.
The pre-wave
The pre-wave runs in both groups before your campaign's first post. It needs time for hosts to accept, for the poll window, and a safety margin, so the quote is refused if your first post is too soon. The page says why.
Any pre-wave poll still open is stopped before your campaign's first post goes up.
The campaign
Your campaign runs as booked, in its own channels only. The matched channels do not run it, and they are chosen so that they have not run your ads recently either.
The post-wave
After each of your campaign's channels has carried your ad for its exposure window, the post-wave poll runs there. The matched channels are polled at the times your campaign's channels typically start, at the same times of day, so both groups answer under the same conditions.
If the campaign moves
If your campaign is moved earlier before you buy, the study is refused or quoted again. If it moves earlier after you buy, the pre-wave polls are stopped before the new first post; if there is no longer time for the pre-wave to run, the study ends and everything, the study fee included, is refunded. If the campaign is cancelled before any reader sees it, the study fee is refunded. See Brand Lift refunds.