That man page isn't the most clear. upvar gives you a way to modify a variable in the calling context (and I think it can go up more than one stack frame).

It's a way to pass by reference rather than the default pass by value (how tcl passes is more complicated than that for performance but the default semantics are pass by value).

It's a facepalm-stupid thing to put into a programming language. Programs that use it will not be efficently compilable; the compiled code will have to provide upvar access similarly in order to make them work.

When compiling a function, you don't know which callee might ask for an upvar, so you can't optimize away variables or keep them in registers across calls.