I struggled with this at first.
Lift the lever to fold the backrest forwards. It now locks forwards but you don't hear any clicks or clunks, it just locks.
Then you can slide the seat forwards, and back. To return it to the old position, slide it back then lift the handle again to release the backrest to vertical where it locks again.
Comprende??
(if you lift the back rest before sliding the seat back it locks where it is, i.e. too far forward)