I don't know of a paper, but on the Tcl wiki there is some discussion of the "non-recursive evaluation" (NRE) engine that enables these features [1]. More formal descriptions of the new coroutine [2] and tailcall [3] commands are listed, among others, here [4].

1. http://wiki.tcl.tk/37253#pagetocbefb5a57

2. http://www.tcl.tk/cgi-bin/tct/tip/328

3. http://www.tcl.tk/cgi-bin/tct/tip/327

4. http://wiki.tcl.tk/21276

Update: more details on the NRE implementation are available here [5] at the contributing author's site; anonymous login required.

5. http://msofer.com:8080/wiki?name=NRE

Tcl's "minor" or point releases are bigger than most languages'. 8.6 has been in development for about four years. 8.5 was also in development at least that long. Contrast Python, which does a point release every 18 months or so.