This is entirely to hard to find the answer to in the kubernetes/kubectl documentation. Apparently the forward is done via socat over tls:

https://stackoverflow.com/questions/50645059/is-kubectl-port...

And socat is mentioned in:

https://github.com/kubernetes/kubectl/blob/master/pkg/cmd/po...

I didn't actually manage to find out what or how socat is called - but I'm also on mobile, so it's a little convoluted to browse source code.

Anyone know where the call to socat happens, after kubectl port-forward?