When you look at a chess set, you see that a Pawn is but a short and scruffy little thing, while a King is tall and majestic. Doesn't it seem strange that a Knight can jump over a King just as easily as it jumps over a Pawn?
In Jack-Be-Nimble Chess, things are different. At the start of a game, a jumping piece can jump over Pawns, but cannot jump over anything taller. After a jumping piece has made one jump, its skill at jumping improves, allowing it to jump over any Pawn, Knight, or Bishop (or pieces of equivalent rank).
Two more jumps allow it to jump over a Rook, and four more (for a total of seven) make it so skillful that it can even jump over a King or Queen.
Of course, most games will end before one piece can make seven jumping moves, and thus one will rarely see an unrestricted jumper; but this is part of the strategy and tactics inherent in Jack-Be-Nimble Chess.
An alternative rule would be to allow jumping over an empty square to count as practice, but I feel that it leads to a more interesting game if you must jump over a piece to advance your skill.
Of the jumping pieces, the Knight is generally considered to move first Rookwise and then diagonally -- in other words, Ng1-f3 jumps over the Pawn at g2, while Ng1-e2 jumps over the B at f1. There is no question about which squares are overleaped by the Alfil and Dabbabah, but the HFD is a special case because it jumps two squares -- how to define it?
I suggest that the HFD is initially able to jump over two Pawns, but not to jump over a minor piece; and that jumping over two pieces at once counts as one jump for skill advancement, the same as jumping over one piece.
An additional variant rule would allow pieces to jump over friendly pieces one rank higher (the friendly piece ducks down to get out of the way). I don't think this rule would add much to the game.
Cannon and grasshopper pieces could also be affected by the Jack-Be-Nimble limitation on jumping.