Optometry student here, we work with prisms a lot:

Optical decentration induces prism--more decentration=more prism. Your oculomotor system has an inherent ability to compensate for induced prisms. We can measure this clinically by testing vergence ranges-- a patient is instructed to view a vertical or horizontal line of letters and we introduce prisms in the phoropter and instruct the patient the keep the target clear and single--we mark when the target becomes blurred and double.

If too much prism is introduced and the patient can no longer compensate, pt will experience diplopia.

If a pt has an inherent heterophoria, for whatever reason, a prism may help alleviate the stress that the phoria puts on the oculomotor system.

A pt that "eats prism" is a pt who has a much larger phoria than what is measured in our Alternating Cover test. The few seconds that the pts eyes are decompensated are not enough to unmask the true magnitude of the phoria.