Apple has used this sort of thing in their laptops in the past, not it seems they want to patent an idea associated with it. When a laptop, or any other mobile device with an internal spinning hard disc drive is rattled, you risk damaging your drive, especially during a read/write cycle.
This patent shows a system that detects when the laptop takes a spill, such as falling off a desk or the user's lap and will detect enough motion to know that something is wrong. According to the patent description, the device that is triggered by the accelerometer will detect a movement. If that trigger meets a certain threshold, perhaps whatever the manufacturer thinks is enough to risk damage, it will take protective action. Likely just stopping the spinning drive and the read/write head.
The patent itself seems to specifically adress minimizing a false trigger of the protective measure. In this case, perhaps the user simply shifting their weight while having a laptop on their lap will not affect the mechanism, unless it detects enough movement to cause a problem.
Description of patent:
Preventative actions normally performed in response to detecting a trigger event triggered by an accelerometer attached to a portable device may be bypassed during a quiet period. The quiet period may be initiated in response to detecting at least one repeated trigger event within a time window encompassing the first trigger event, and may be terminated based on an expiration criterion.