When you're talking about lithium-ion batteries, the word "pyrotechnic" is not usually considered a good thing. But two new patents filing by Tesla shows that it's not always a scary thing to have such an explosive term connected to your power source. The wording in the filings is, as always, a bit complicated but the takeaway points from patent #20170229266 and #20170229268 (they share similar wording), is that in the event of a crash, the high-voltage battery in an electric vehicle needs to be neutered so that it's safe for anyone who might come into contact with it. Or, as Tesla says in a filing:

 In demanding applications, the disconnect must interrupt very large currents in a fast and reliable manner. For example, the interruption of large currents has a tendency to create electric arcs, sometimes referred to as arc columns. Since the disconnect is often intended to improve safety of the electric system (by allowing it to be disconnected quickly), it is important that such electric arcs are then managed so as to not create a new hazard or risk further damage. At the same time, it is preferable that the disconnect not be overly complex or involve components that are unduly expensive.

Disconnecting a battery in the case of danger is vital to keep people safe. Tesla says its "pyrotechnic disconnect" can be used in either an EV or in stationary storage. So, how does it work? Well, there's a housing with a combustion chamber in it, and there's a pyrotechnic charge inside that combustion chamber and a busbar covering on the outside. When the busbar is severed, for whatever reason, there's an exhaust port in the housing that lets gas escape so that any electric arc created is suppressed. These arcs can be a problem because they can, "create a new hazard or risk further damage." With Tesla's idea here, the electric system can be disconnected quickly, and the arcs can then be managed.

Source: USPTO (1, 2)