In order for it to be Quan (the fist) Master Li said that the Yi (containing in the interior) can be released externally and the Xing (releasing in the exterior) can be contained internally. But the next question is what makes Xingyi Xingyi Quan. Now this is basic Xingyi theory that every student learns within the first few lessons. On the interior the Shen harmonizes with the Yi, the Yi harmonizes with the Qi, the QI harmonizes with the Li. On the exterior the hand harmonizes with the foot, the elbow harmonizes with the knee, the shoulder harmonizes with the hip. The harmonies are another theoretical guide. Now the 'six harmonies' are a way way to help with understanding this. Cheng Quangong then quotes Master Li saying, basically, that containing in the interior is Yi and releasing to the exterior is called Xing. Now this is true in a theoretical sense but it doesn't express what you should really feel in the body after a long practice of Xingyi. You can read the responses on this site but basically they just say that Xing means form or shape and Yi means intent or mind. Early on he asks people to explain what they think Xingyi Quan means. On this site you can find a lecture given by Master Li's disciple Cheng Quangong.