The kill switch is a safety net: if the VPN tunnel drops unexpectedly, the kill switch keeps your device offline until either the tunnel recovers or you turn the VPN off explicitly. No plaintext leakage in the gap between drop and reconnect.
Turn it on (iOS)
Open Settings inside the app → Connection → Kill Switch → Enable. iOS implements this via NEVPNManager's on-demand rules — the system itself enforces it, so it survives backgrounding and crashes.
Turn it on (Android)
Open Android system Settings → Network → VPN → SecureFox → cog icon → Always-on VPN + Block connections without VPN. SecureFox detects these toggles and shows a green Kill Switch active badge on the home screen.
What it doesn't block
Local-network traffic (your printer, smart TV, NAS) keeps working — the kill switch only blocks outbound internet.