  We address the microscopic origin of the universal three-body parameter that
fixes the spectrum of three-atom systems in the Efimov regime. We identify it
with the van der Waals two-body correlation, which causes the three-atom system
to deform when the three atoms come within the distance of the van der Waals
length, effectively preventing them from coming closer due to the
kinetic-energy cost associated with this three-body deformation. This
deformation mechanism explains the universal ratio of the scattering length at
the triatomic resonance to the van der Waals length observed in several
experiments and confirmed by numerical calculations.
