Stage1:
The first group of every third slat/plank moves under the load. (Load does not move.)

Stage2:
The second group of every third slat/plank moves under the load. (Load does not move.)

Stage3:
The final group of every third slat/plank moves under the load. (Load does not move.)

Stage4:
All Slats/planks move together.
Load moves toward the discharge end.