When last I checked (with recipe http://fold.it/portal/recipe/46844 DoBandsObeyHookesLaw1), Foldit bands obeyed Hooke's Law for springs: F = -k x, where k is a spring constant, x is the distance the band has been stretched from its equilibrium value, and F is the force exerted by the band. The - sign is there to tell the direction of the force F (the force is always opposite to the direction the band is stretched or compressed from equilibrium, forcing the inter-atom distance back towards equilibrium). These bands behave like springs attached at both ends to the protein.
Would it be possible to add a new class of bands that act like springs attached at just one end to the protein? One kind would behave like bumpers on cars. If the band gets compressed below its equilibrium distance, the band behaves like normal and obeys Hooke's Law, forcing the distance back toward equilibrium. Nevertheless, if the band is stretched beyond its equilibrium distance, the band exerts no force on the protein.
Another kind of band would obey Hooke's Law when stretched beyond its equilibrium distance, but exerts no force when compressed below its equilbrium distance.
These new kinds of bands would help with Contact Map puzzles (see http://foldit.wikia.com/wiki/Contact_map for example). With two of these bands, one could set the minimum and maximum distances between two atoms on the protein. As long as the atoms stay between these minimum and maximum distances, the bands behave as if they are not present, but once the atoms move out of the desired minimum to maximum range, the atoms will feel Hooke's Law forces like from normal bands, forcing the distance between the atoms back into the desired minimum to maximum range.