Originally posted by David113
Release the position (Retract moves until you get a position which is obviously legal).
2wR and wB were captured behind their own pawn wall. Ba7 is not promoted - wP\'s prevent a promoted dark-square B from leaving the first rank. It must go home before ...e7 is retracted. 1...Qc8xNd8?? leaves no way to ever unlock the NW corner.
So, how to shield the 8th rank for wK? Only a bNd8 can possibly work, and it must come from c6. However, an N of either color cannot retract directly to c6, which would have one of the K\'s in check with checking side to move.
The only answer is to add a non-N piece to the mix, outside the bP and wP walls, and use it to deliver an N on c6 legally.
White needs to uncapture a bNg8 right away to give Black some moves, which makes 3 bN\'s needed, so one is promoted. A promotion requires at least two captures by the a-pawn, one of which must be a wN. White must therefore wait until this wN is back on the board, so it can uncapture the non-N, which uncaptures yet another wN on c6, which uncaptures a bNc6.
Retract 1...Qc8 2.Qh8xNg8 Nf6 3.Qg8 Ne4 4.Qh8 Nc5 5.Qg8 Nb3 6.Qh8 Na1 7.Qg8 N=a2 8.Qh8 b3xR 9.~ a4xN 10.Nd4xRb3 Rc3 11.~ Rc6 12.~ RxNc6 13.NxNc6 Nd8 etc.
With wK freed, the position unwinds easily.