AMBER also uses improper torsions to enforce chirality.
A definition from http://www.amber.ucsf.edu/amber/newparams.html:
Improper torsions are so named because the atoms involved are not serially
bonded; rather they are branched:
They are additionally used in the united-atom force field model when a
carbon with an implicit hydrogen is a chiral center; in effect they keep the
position from inverting (PN=3).
Is anyone still refining united-atom force fields?
My impression is that they may be 'old technology' now.