Let's say you're surfing along and suddenly your VPN connection fails. Your phone or computer is likely to immediately try to reconnect and do so directly without going through a VPN. All of a sudden, your data is unprotected.A kill switch is a feature in your device's VPN app that detects when a connection fails and immediately shuts down network access. Like with everything, it's not a 100% perfect solution, but these days, we wouldn't recommend using a VPN that doesn't offer a kill switch.