It is obvious (to sound engineers at least) what the effect of cupping a handheld microphone does to the polar response of directional microphones in live performances, which is popular in a lot of the metal genres, but DPA have gone and published an article about the effects on frequency response too, including a resonant bump around 2kHz. Thanks DPA! Here is the full article on the DPA website:
https://www.dpamicrophones.com/mic-university/proper-hand-placement-on-a-vocal-mic