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Greetings, gentlemen! Recently, I purchased a BMW 640d and have seen an unusual jolt in my seat upon the vehicle's shutdown. I have a strong affinity for the automobile, nevertheless, I am perplexed as to why it exhibits a peculiar behaviour at engine shutdown, namely delivering a forceful impact to my posterior region.

I have also seen this phenomenon and would much appreciate an explanation on its underlying origins.

Is it possible that the seat belt is the cause? Is it possible that the tension applied to the seatbelt roller, located below the seat, is responsible for removing any slack in the seat belt?

What are the potential solutions to address this issue?

Prior to attempting to rectify the situation...Initially, I would ascertain if it may be the source of the problem.

If you are currently occupying the seat...Is there any sensation perceptible when you retract the seat belt by pulling it and then releasing it?

I mentioned it as a potential explanation since there is an object under the seat that has the ability to move independently when the seatbelts with active "hug" feature are engaged...This might perhaps be a viable option...Particularly if the sensation you experience is linked to the motion of the seat belt.

The sensation seems to originate just under the centre of the seat. The seatbelt tensioner is a very probable option. Additionally, it is worth noting that the issue occurs concurrently on both the driver and passenger seats.

I do not believe that it requires any repairs. I only like to be informed about its functionality, and the seatbelt tensioner seems to be a feasible solution.

It now seems to be sensible. Thank you for providing the answer.

Discovered the issue, however it remains unresolved and far from being resolved.

Origin of my analogous issue

My chil