It's probably not going to wait to add fuel until the ECU detects boost. That would be too slow, because it takes time for fuel to make its way into the combustion chamber. Chances are, the fueling will be ramped up in anticipation of boost, to make sure the engine doesn't go lean.
Also, I hate to be a dork but maybe this convo belongs in a different thread. We're wandering too far off topic.
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