QMOT | Orbital mixing in methyl

The following picture is from E. V. Anslyn *Physical Organic Chemistry* and shows constructing of MO of methyl using QMOT. MOs $\textbf{E'}$ and $\textbf{D'}$ are formed by mixing $\textbf{D}$ and $\textbf{E}$. I can't quite imagine how the mixing goes here. 


[![Walsh diagram for for CH_\sub{3}](https://i.stack.imgur.com/fA7XQ.png)](https://i.stack.imgur.com/fA7XQ.png)


