Who said it is coordinated. I used to do a "knife edge" slip all the way down, purposefully uncoordinated. Let me see if I can find the pictures of that. See my post no. 4 on the thread below for an explanation of the knife edge steep spiral. If done correctly it is the fastest way down and was/is used for decades by various drop zones. Structural issues arise when pilot technique is shoddy, and I would not recommend doing this maneuver without a g meter even with a decent pilot. You want to push 3 g's and no more, but poor pilot technique can lead to over g'ing the airplane.

Knife edge thread