When it is mounted in its mounting bracket under the seat, you unscrew it so the jack extends outward enough to hold the jack tightly between its two mounting points. When properly extended, the jack also serves to secure the jack handle and prevent it from rattling.
This is the best photo I have found which shows its mounting point. But IIRC, the owner has the below jack rotated 90 degrees from its proper orientation where the knob the jack handle fits into should be pointing straight up. That orientation allows it to lock into its bracket and gives good access to the knob so it can be turned to extend the jack so it locks firmly in place between its mounting brackets. I'd take a photo of mine but the shoulder harnesses that pass through the seat prevent me from moving the seat forward enough to expose the jack.
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