I thought it's a simple mnemonic: -> means inserting the form just after the beginning of the next one, while ->> inserts at the end of it.

You most certainly can have explicit variable to thread through expressions, like this:

   (it-> (form1 it) (form2 3 it 4) ...)

where it means "previous expression". I think there's a Clojure library which already implements this. You can use that if you want.

in Clojure this is achieved with `as->`

In Racket we actually have a threading macro with ~>

This:

(- (bytes-ref (string->bytes/utf-8 (symbol->string 'abc)) 1) 2)

Becomes this:

(~> 'abc

    symbol->string

    string->bytes/utf-8

    (bytes-ref 1)

    (- 2))

https://docs.racket-lang.org/threading/index.html

I believe the use of "~>" rather than "->" in Racket was set in #lang rackjure which adds in some of the Clojure syntax to Racket. Also amusing is their explanation:

> For example the threading macros are ~> and ~>> (using ~ instead of -) because Racket already uses -> for contracts. Plus as Danny Yoo pointed out to me, ~ is more "thready".

http://docs.racket-lang.org/rackjure/index.html#%28part._.Ba...

More thready indeed :-p

Emacs 25 has thread-first and thread-last macros in subr-x.el.