I generally do it before I add the yeast. Depending on how well you can control the injection rate you can create a lot of foam that rapidly runs up the neck of a carboy. I'd rather not be blowing the yeast out of the fermentor.
I inject for about 60 seconds or so. If there is a lot of foam I give it a couple minutes to settle, then I pitch the yeast and swirl the carboy to mix them in.
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