It is from the beginning of Q1.Given that it's standard for teams to change engine maps on the fly I don't see the point of this particular rule.
What's stopping them driving with the hot-blowing engine map enabled for most of the lap then switching to a normal engine map for the last corner and start finish straight.
Scarbs seems to believe drivers can alter the engine map after lap 2 of the race, so we have conflicting information. If you can do it via the steering wheel, it won't be that effective. If you can only do it during a pit-stop and then only by connecting a laptop well, then it can be somewhat effective.
We'll see what happens or if more info comes out.