actually myofascial release practitioners do a very similar manouvre for plantar fascitis- iI did a course this year and the leader described a similar technique - if you appreciate that the body is interconnected fascia then a sustained stretch in this manner obviously gives the plantar fascia a good stretch...have a look at some myofascial release texts if you can as they detail the techniques you can use. Sadly there isn't a great deal of research out there at the moment!