White parts tend to turn blue disk of 4 pins and to go down due to their weight.
Red spring forces pink pawl towards blue ratchet wheel to prevent the disk from rotation.
Once the part A overcomes the blue disk, the latter stops the next part B. The part A goes down and pushes violet lever to pull the pawl from the ratchet wheel to let the disk turn 90 deg. and the part B can go through.