The photoresist molecules would be activated by coherent population trapping (CPT), a process used in slow light and other atomic experiments. In the simplest case, two lasers drive two transitions from different lower energy states to a common excited state, but due to a quantum interference effect, the molecules are never excited. Instead they evolve into a so-called dark state--a stable combination of the lower states that is unaffected by light. With additional upper and lower states, there may exist more complex dark states that combine several low-energy states and that could be populated using additional lasers tuned to the different transitions. CPT does not require multiphoton absorption, so it can work at relatively low intensities.
This is such a clever scheme. Of course, there's still a long way to go before such a technique can be used in an industrial setting.