From what I can tell, the channel opening is covered by the car panel. The one locking point is at the end of the channel, closest to the floorline of the car. It's free to slide backwards and forwards by several mm when unlocked like this. Because the channel is closed off by the car panel piece which makes up the inside and front of the bicep, you can slide the bicep this way and the arm will never fall off, but I don't think it's intentional. Just a happy acident if you think that the tradeoffs are worth it.