Say pieces on a board, make each a pair with another piece.
so figure out how a piece can move.
pick any piece, try to move it somewhere.
when you move a piece you have to move it's pair at the same time.
when you move to a piece it's pair has to move at the same time too.
a piece always becomes a pair with the piece it moves to.
no matter how many pairs, there's only one answer to how a piece can move.
A common problem, I forget what it's called.
There's only one answer for how any piece can move.
A piece always goes where a piece leaves.
No piece can move to where a piece moves back where it came from.
No such thing as a free space, a piece always moves to another piece.
A pair never moves to a pair.
so try this...
draw for each piece a line from one piece to another that connects
each piece to move from the first piece until the last piece that goes
back where it starts.
see this as a machine diagram.
move a piece then figure the machine diagram again, it's the same machine.
that's a machine getting work done...