I don''t know if they expect it to fit only a NB shoe or something (mine''s Asics), but mine fell off the first time I used it this morning.  I had to back track to find it.  It doesn''t have enough contact and friction to stay on the rear of the shoe when you are pounding down the road.  I guess you could wear it around your ankle.  I wore it just above the elbow for the remainder of the morning run.  In either case you will also have to tolerate the pinching pressure on you body.

Worked great for a turn back the clock nighttime 5k