Split tunnelling
By default a tunnel takes every bit of the device traffic: the browser, the mail client, background updates. Split tunnelling lets you set exceptions — by application, by address or by domain. The point is not saving bandwidth but that some services work better, or only work, on a direct connection: a banking app may refuse an unfamiliar address, a local printer and smart devices live on the home network which the tunnel does not reach, and a game client picks its nearest server by latency and through a distant exit picks the wrong one.
In short
A sensible setup usually looks like this: the browser and whatever the connection was set up for go through the tunnel, while home devices, banking apps and the local network stay direct. The main trap is forgetting that an excluded app runs from your ordinary address: put the browser on the exception list and expectation and reality part ways silently, without a single error message.
Frequently asked
Where is split tunnelling configured?
In the client app itself: most offer a list of apps or addresses with a mode switch — only these through the tunnel, or everything except these. On a router the same is done with rules for specific devices, and then the decision is made once for the whole home network.
Why is my home printer invisible while connected?
Because requests to it go into the tunnel, and the tunnel leads outward — they never reach a device on your own network. That is exactly the case exceptions exist for: home network addresses are routed outside the tunnel, after which printers, cameras and speakers become visible again.