Some discussion here :- **LINK**
Multistep process seems bit long winded but does, theoretically, avoid over-burning the hole.
Theory of the spring is that it forces the softwood handle around the final part of the tang under the ferrule giving a good grip without needing to burn the big end of the hole to shape.
Sort of thing you need to do to make a cheap, relatively crappy, handle work. Back in ye good old days with proper turned brass ferrules the handle cost around half as much as the file so you looked after it and expected it to do several files. Of course a file was a days wage or so too. Was years before I could justify new files as opposed to reconditioned ones via Mr Whistons (in)famous Cat on its log.
Clive