The turbulent wake of a jumper. Drift into it and you lose drag and control in an instant — 'you were in my burble' is never a compliment.
The burble isn't a fringe phenomenon — it explains a good share of the chop in group jumps: behind an unstable flyer the airflow separates, and anyone who drifts in there simply has less air underneath them for a moment. The right answer isn't to fight it but to make space — in freefly the rule is to move away from unsteady flyers rather than closing in, and to correct fine rather than gross. One misconception that tags along: the burble isn't behind you in the sense of where you are facing, it is above you — in freefall the relative wind comes from below. And another, specific to wingsuits: for the opening, your speed at the moment you pull counts as the bigger risk factor, not the burble.