在拳則為橫。橫者卽拳中先天圓滿中和之一氣也。

內包四德。即劈崩躦砲也。

亦即真意也。

- 李奎垣先生

In that quotation above,  Master Li Kuiyuan is saying that Crossing is the core and essence of Xingyi and that it subsumes the other techniques. You might say: huh why dat? In my book AXE Advanced Xingyi Energetics I explain the deep inner meaning of this. It isn't what you think (that the outer full body technique within the 5 Elements called 橫拳 is the thing). Nope, it has a more inner significance. Notice he doesn't say 橫拳, he says 橫 or 橫者 - but don't worry, it's  not explained as some philosophical goop but rather something you can practice for yourself in every 'Fist' of Xingyi, to infuse your forearms and hands with internal power. Check the book out.