Or do they use the main sensor for the EV ?

This is made possible by adopting an approach that has more in common with a mirrorless camera (like the Panasonic G2, for example) than an SLR by removing the bits that pretty much define such cameras: the optical viewfinder and moving mirror.

The designation 'SLT' stands for single lens translucent and it's the 'translucent' bit that's the key to what differentiates these new models both from conventional DSLRs and mirrorless interchangeable lens cameras. The SLTs do have mirrors, but they're mirrors that let the majority of the light pass straight through to the sensor, rather than having to swing out of the way to allow exposure. As a result they are fixed in position, always reflecting a portion of the light emerging from the back of the lens onto a phase-detection AF array housed in the top of the camera. (A newly-developed 15-point array in the case of these two cameras).