At SHA2017 ( a hackers camp in the netherlands), we explained how the timestretcher performs its work without altering the pitch of the track.
The presenter is out of breath at the beginning of the talk because he just did a quick dash to his tent and back; under the assumption that another portable would have been necessary to project the slides. That turned out not to be the case, but still, our apologies for this.
Slides of the talk can be found at http://werner.yellowcouch.org/Papers/sha2017/.
The article can be found at http://werner.yellowcouch.org/Papers/zathras15/.
A youtube version of the talk is at https://www.youtube.com/watch?v=mQwot1HF2cU