A kill switch is the part of a VPN app that cuts your internet connection the moment the encrypted tunnel drops, instead of quietly letting traffic fall back to your ordinary connection. It exists because a VPN failing is not a dramatic event: the app keeps its icon, the page keeps loading, and the only thing that changed is that your real address is now attached to everything you do. The kill switch turns a silent failure into an obvious one.
What actually happens when a tunnel drops
VPN connections drop routinely and undramatically. A phone moves between wifi and mobile data, a laptop wakes from sleep, a server is rotated out for maintenance, a network hiccups for four seconds. Without a kill switch the operating system does the sensible thing and routes your traffic down the ordinary path, so the download continues, the video keeps playing, and nothing on screen suggests anything has changed. That gap can be seconds or hours depending on when you next look at the app.
The two kinds, and why the difference matters
A system-level or firewall kill switch blocks all traffic at the network layer until the tunnel is back, so nothing on the device reaches the internet in the meantime. An application-level kill switch closes only the apps you have listed, which is gentler and leaves everything you did not list connected. The first is the one people mean when they say kill switch; the second is useful if you want one specific application stopped and the rest of the machine online. Providers use both names loosely, so the setting screen is worth reading rather than the feature list.
How to check yours works, without waiting for a failure
Connect the VPN, start something that holds a connection open, then quit the VPN app abruptly from the operating system rather than using its disconnect button. With a working system-level kill switch the connection stops dead until you reconnect. If the page carries on loading, the switch is off, scoped to applications you did not list, or not present on that platform. Mobile apps are the usual place it is missing, and platform coverage is stated on each provider's own page rather than in the marketing summary.
It is not a niche feature and it is not a paid extra
Every provider in this index that publishes a feature list includes a kill switch on its standard plan, and none charges for it separately. That makes it a poor reason to choose one provider over another and a good reason to check the setting is actually enabled, because on several apps it ships switched off. The comparison that does separate providers is the one this site measures: the printed price, the prepaid term and what the plan renews at.
Questions people ask about what is a vpn kill switch
Should I leave the kill switch on all the time?
On a device you use for anything you would rather not attach to your own address, yes: it costs nothing and turns a silent failure into a visible one. The usual reason people turn it off is that a system-level switch also blocks the local network, which can interfere with printers and casting.
Do free VPNs have a kill switch?
Some do and some do not, and free tiers are where it is most often absent on mobile. The provider's own feature page states it per platform, which is the page to read rather than a review.
Does a kill switch hide the fact that I use a VPN?
No. It stops traffic leaving outside the tunnel; it does not disguise that a tunnel exists. Those are different problems with different answers.