At the end of the function dplyr::bind_rows, the function dplyr:::bind_rows_ is called. The full code of the this function is:
function (dots, id) 
{
    .Call(`_dplyr_bind_rows_`, dots, id)
}
What does this function do?
It runs the C function _dplyr_bind_rows_ with the arguments dots and id. 
For more information about using .Call in R, see for example these slides.
