Plain English Meaning
The part of the turn where the airplane is heading the same way the wind is going, so the wind is at its back and it is moving across the ground faster.
Full Definition
The portion of a turning maneuver during which the airplane is pointed in the same general direction as the wind is blowing, so the wind is pushing the airplane along its track. During a steep spiral, the segment flown on downwind headings produces the highest groundspeed and the widest ground track, requiring a steeper bank to hold a constant radius around the reference point.
Context Anchor
Encountered when flying a steep spiral or any ground-reference maneuver where wind changes the airplane’s path over the ground.