That is the binding return spring. One on each binding. Very optional maybe the only good reason I could think is for side stepping which is better accomplished by locking heel. Other wise it adds weight and takes a tad more energy to lift boot when skinning.
On the subject of return springs and binding design, here is an old post that illustrates Chuck Loughney's Snowstar, a locally designed binding from the 1980s:
www.turns-all-year.com/skiing_snowboardi...ex.php?topic=11336.0
Loughney figured out how to incorporate a return spring without some of the disadvantages mentioned here.
The progress in backcountry skiing gear in the last 30 years is pretty amazing to those who've been at it that long.
