Slat retraction (during which the load does not move) is accomplished by moving only one set of slats at a time. (The friction of the load on the two stationary sets of slats keeps the load from moving while a single set of slats alternately slides past.)
Slat retraction involves moving one set of slats at a time, utilizing the static friction between the stationary sets to keep the load in place as each set glides past.