Hi Peter,
It's called a "hera." In certain ways of wearing hakama, the obi gets tied in the back (rather than in the front as seen most commonly in aikido). The "hera" then gets tucked in between the obi while the koshiita then goes above the obi knot.
You can see this tucking in the sixth image on this page:
http://www.geocities.jp/ikushinkai_p/o4.html
-- Jun