No compression or imitation of such. An electric field is used to manage the entropy of the material, and in a system of constant volume, entropy is directly proportional to heat.
The exact mechanism and the associated equations are a little beyond me, so I can be wrong on the rest, but if I got it right, the idea is to reduce entropy in part of the material, so the heat immediately flows into this part to get entropy balanced again, but by doing so it is moved from everywhere else, effectively cooling the other part of the material down.