[Click] how the push and pull implement in the bottom code

[kele kele]

 Hi,
       I'm doing some reash on click. Now a problem confuse me, how the
push and pull implement in the bottom code in click. I have tried to find
 the relate code in click sorce code, but I'm failed. Can anybody help me,
tell me the mechanism of push and pull in click and the related code
location in click source code. Very thanks.

                Kelei Jin