It’s a head car with automatic hook (Mod. HS24.210), which hooks onto special stops (Mod. HS24.17 or HS24.18) on the mast track to unload the halyard.
Two manoeuvres from the hook go down along the sail: 1 - to disengage the hook and allow hte car to lower. 2 - to load the hook, which hooks onto the stop on the track.
For further information and details on the topic, we recommend this article.