Optical patch nano-antennas possess unique absorption, field enhancement and concentration capabilities - but their crosssection, as well as their response outside of normal incidence are not well understood. Here we explain the large cross-section by considering that each patch nanoantenna is a cavity excited from both sides. Such a simple physical picture allows to fully understand the influence of the angle of incidence - that odd resonances have a very high absorption cross-section which decreases when the incidence angle increases, while even resonances cannot be excited in normal incidence. A direct application would be to use these structures as an optical nanometric set-square.