A kill switch is a safety feature built into VPN applications and some networking devices. When your VPN connection fails unexpectedly, the kill switch automatically disconnects your device from the internet entirely, rather than allowing your traffic to fall back to your regular unencrypted connection.
This matters because without a kill switch, a sudden VPN dropout could expose your real IP address and browsing activity to your internet service provider, website operators, or other observers for a few seconds or longer. The kill switch prevents that window of exposure by stopping all network traffic immediately.
Kill switches come in different strengths. A basic kill switch simply cuts off your connection. An advanced version, sometimes called a "network lock" or "internet kill switch", can be configured to allow only certain applications or traffic types through, or to auto-reconnect to the VPN once it comes back online. Some operate at the app level (stopping only that VPN client's traffic), whilst others work at the system level (blocking all internet access across your device).
Real-world example: you are downloading a file over your VPN connection when your VPN server becomes unreachable. Without a kill switch, your device reconnects to regular internet and the remaining file chunks download in plain view. With a kill switch active, your internet cuts off immediately, and you must manually reconnect the VPN before downloading resumes.
When choosing a VPN or networking tool, check whether a kill switch is included and whether you can customise its behaviour. Look for clarification on whether it operates at the app level or system level, and whether it supports auto-reconnect. A kill switch is especially valuable if you value privacy during file transfers or P2P activities.