  Numerical simulations have shown that a recently proposed 'crabbed waist'
scheme of beam-beam collisions can substantially increase the luminosity of a
collider. In this paper we give a qualitative explanation why this scheme
works. For this purpose we use simple geometrical considerations and analyze
peculiar properties of vertical motion modulations by synchrotron and
horizontal betatron oscillations. It is shown that in the 'crabbed waist'
scheme these modulations, which are the main sources of beam-beam resonances
excitation, are significantly suppressed. Some numerical examples demonstrating
the effect of the crabbed waist collisions are also given.
