As far as Metamachy is concerned, you guys loose me. I don't understand what you say "And the jumped-over piece would then block its protector from moving to the e.p. square to make the capture".
There is nothing exceptional with the King's jump in Metamachy. It is simply to adopt the rule that was in force at chess in several places in the Middle Ages, before castling was adopted. Basically, the King was allowed an initial 2-square away move, including horse's leap, providing he was not in check and he was not passing over a square where he could have been checked. The same idea is still applied for castling, the King must not go over a square under threat either. Castling is a particular case of the King's jump. Plus, of course, the fact that castling gathered two consecutive moves in one, 1) bringing the Rook close to the King, 2) the King jumping over the Rook. That is the true evolution that led to our modern castling.
As far as Metamachy is concerned, you guys loose me. I don't understand what you say "And the jumped-over piece would then block its protector from moving to the e.p. square to make the capture".
There is nothing exceptional with the King's jump in Metamachy. It is simply to adopt the rule that was in force at chess in several places in the Middle Ages, before castling was adopted. Basically, the King was allowed an initial 2-square away move, including horse's leap, providing he was not in check and he was not passing over a square where he could have been checked. The same idea is still applied for castling, the King must not go over a square under threat either. Castling is a particular case of the King's jump. Plus, of course, the fact that castling gathered two consecutive moves in one, 1) bringing the Rook close to the King, 2) the King jumping over the Rook. That is the true evolution that led to our modern castling.