When you want to reposition your Flexx, just place your foot in the space under the foot surface and lift it up. 

Put it down on the new place and remove your foot.

Place Flexx on the floor in the usual place as for an ordinary footrest, i.e., just ahead of your chair.  Taller people may prefer to position Flexx slightly further away: shorter people, closer.

Put your feet on it and rest!

Move Flexx back a bit and notice how it supports your feet slightly differently - move it forward slightly, and it will be different again!

Try a few spots for a while each.

Even before you start using your Flexx dynamically, feel how it provides soft support for your legs.