It was a "pyrotechnic battery disconnect", i.e., basically a type of switch/active fuse using some type of small explosive charge as part of its operation.

Here's the patent for all the gory details:

https://patents.google.com/patent/US9221343

The fact that this part has a "problem" probably means that it was triggered, e.g., maybe some type of false collision detection, short circuit or other failure which ended up triggering the disconnect.

"Disconnect" also belongs to the expression - essentially, a tiny explosive is used to ensure that the safety switch disconnects the battery from the load side, permanently and without arcing: https://insideevs.com/news/334322/tesla-patents-pyrotechnic-...

A pyrotechnic battery disconnect, which I suppose cuts the connection to the battery instantly in case of accident to avoid shorting it. There are already pyrotechnic devices in cars: airbags are pyrotechnic, as well as safety belt detents. It's the fastest type of actuator.