No, say v = "MyFrame" - this is a string with the text of a frame name.
v = _G[v] looks up "MyFrame" (the string) in the global table and sees that it is the global name of a frame object. The variable v is then set to the actual frame object. In other words, whatever the right side evaluates to, the left side will be set to.
You can then do v:SetAlpha() because v is referencing the frame itself, not the frame name.
/edit: You think the above works backwards from how it actually does. You're replacing the entry in the global table with a string. You're breaking the whole UI.
