The car presence sensor is the built-in barrier position sensor. This sensor detects when the barrier is pressed down to the ground; that in turn only happens when the barrier is driven over.
Assuming that the barrier is installed in the middle (depth-wise) of the stall, when a car pulls in only the front tire drives over the barrier. That pushes the barrier to the ground once. At this point the MySpot 200A is dormant and will not obey commands or rise (same as standard MS200).
When the car pulls out, the front wheel engages the barrier again and pushes it to the ground. This starts a timer and 20 seconds later the barrier will rise. If the front overhang of the car (the area in front of the wheels) is still over the barrier after 20 seconds (e.g. the driver is waiting for traffic to pass by), the barrier will rise, hit the undercarriage of the car (gently) and immediately go down automatically (because it did not reach the full locked-up position within 1 second).
If the driver pulls back into the stall within the 20 seconds time window, the cycle is aborted and a new entry cycle begins.
