  We study the non-linear evolution of a dust ellipsoid,embedded in a Friedmann
flat background universe, in order to determine the evolution of the density of
the ellipsoid as the perturbation to it related detaches from general expansion
and begins to collapse. We show that while the growth rate of the density
contrast of a mass element is enhanced by the shear in agreement with Hoffman
(1986a), the angular momentum acquired by the ellipsoid has the right magnitude
to counterbalance the effect of the shear. The result confirms the
previrialization conjecture (Peebles & Groth 1976; Davis & Peebles 1977;
Peebles 1990) by showing that initial asphericities and tidal interactions
begin to slow down the collapse after the system has broken away from the
general expansion.
