What was uke doing while you were manifesting your intent? Was he just standing there passively or giving you the energy of his attack? What was he attacking with; grab or blow? Have you tried the same experiment with an inanimate object?
Fist pressing into guts. Works also on a broom wedged into guts (at least I can replicate the feeling - broom handle does not actually go anywhere). I use a push broom with the head in my guts and the handle wedged against a stop on the floor. The wooden handled push broom flexes - the metal one not much except the feeling is there.