Dear
Su qi,
This should be explained in:
Brandes, U., & Mader, M. (2011, September). A quantitative comparison of stress-minimization approaches for offline dynamic graph drawing. In International Symposium on Graph Drawing (pp. 99-110). Berlin, Heidelberg: Springer Berlin Heidelberg.
If not in that paper, then certainly in the dissertation of the second author:
Best wishes
Juergen