Sparring
You start in an ending from a real game, a pawn or two up, with the move. No lesson and no script: play it out against an opponent set to your strength. Every move gets a verdict as it lands, and a mistake comes straight back with the refutation drawn on the board, so you see what it would have cost before you play on.
Four kinds of ending
117 positions from real games
Each door deals the next position of that kind. The board on it is one of them.
Queen endings
Queens still on, and one side a little better. Checks are everywhere, so conversion is about shelter for your own king as much as pressure on theirs.
22 positionsPlay oneRook endings
The most common ending there is, and the one most often thrown away. Activity beats material here more than anywhere else on the board.
31 positionsPlay oneMinor-piece endings
Bishops and knights, where the right trade decides the game and the wrong one draws it.
35 positionsPlay onePawn endings
No pieces left, so nothing is approximate: every tempo either wins the game or loses half a point.
29 positionsPlay one
Checked before they land here
Every position came out of a real game and was checked before it landed here: the side you play is genuinely better, the edge is still there after best play from both sides, and the position is not already decided. Some of them are proved wins, confirmed by the tablebase or by the engine converting them against itself. Most are the commoner and more useful thing: an advantage that perfect defence would hold, and your opponent is not perfect.