Hi,
Circuit under the pad (CUP) essentially means the same as it sounds, mainly done to save area in dsm processes usually, the drivers and esd protection structures are placed under the pad. But the bondpad must be able to handle the stress at the same time so there are special structures the via placements done in groupings and stacked to tackle the stress under the bondpad, some foundries call it ' copper line via support structures' clvs structures which are added to tackle mechanical stress. In geometries like 45nm the entire logic + esd + drivers etc all come under the bondpad...essentially the entire pad is the bondpad in whole !!