I recently got redirected to this website on sharing a bit of research I did, and while I'll be doing my best to catch up (after starting at page 1 of this thread), I wanted to share my findings already.
Essentially, I came upon the post of a TIF who mentioned her sausage, which is grafted from a leg, makes her feel like she's touching the graft site when she touches this fake dick. This got me thinking - and while I can't find anything useful (or genuine) in regards to arm- and legcocks TIFs get sewn on, I did find a very similar situation in toe-to-thumb transfers. It's a bit of a read, but it all boils down to the body recognising part from a graft site as still being connected to that site. Essentially - their fake dicks don't just look like a piece of an arm or a leg, but the TIF experiences them as being part of it.
Apologies if this train of thought has been posted before, but if not - I hope it's useful.
My hypothesis and answers I found, with sources listed at the end, and screencaps of text in source throughout: