VPN basics · Plain-English answer
A VPN kill switch is a safety feature that blocks all internet traffic if your VPN connection drops unexpectedly. Without one, your device would quietly fall back to your normal connection and expose your real IP address. With it, nothing leaves your device until the tunnel is restored.
VPN tunnels can drop for ordinary reasons, such as switching from Wi-Fi to mobile data, a laptop waking from sleep, or a brief server or network interruption. The dangerous part is that most operating systems immediately resume traffic over the regular connection, so for a few seconds or minutes your real IP address and unencrypted DNS queries are exposed without you noticing.
A kill switch removes that gap. It watches the tunnel and cuts network access the instant the VPN is not protecting you.
Turn the kill switch on if the reason you use a VPN would be undermined by even a brief exposure. That includes torrenting, working from networks you do not trust, and any situation where hiding your real IP is the whole point.
The trade-off is that a dropped tunnel means no internet until it reconnects, which is mildly inconvenient but is exactly the guarantee you asked for. ROOT VPN includes a kill switch in its apps, and pairing it with a leak check at /tools/dns-leak-test/ confirms your setup holds together.
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