("advance" it), and to translate the condyles (open the mouth).
(Although one lateral pterygoid can contract without the other, the result is
a swinging of the mandible to the opposite side, and serves no functional purpose.)
(The image-link above take's you to a Flash Animation of the condyles
translating.
The lateral pterygoids are not shown, but their effect is)
The contraction of only one of the lateral pterygoids (when the teeth
are apart)
will swing the mandible to the opposite side, and is a purposeless
act.
Once the jaw has been elevated (by the temporalis,
green arrows)
and there is a tooth contact (target), contraction
of the lateral pterygoid(s)
is parafunctional. During
function (chewing), there is no purpose in the attempt to
translate the condyle. The intensity of the parafunctional
contractions will dictate
the degree of resultant signs and/or symptoms,
if any.