What is the reason behind the stereoselectivity of the conversion of vinyl silanes to alkenes?

I have included some snapshots from clayden organic chemistry:
[![enter image description here](https://i.stack.imgur.com/rCxcQ.png)](https://i.stack.imgur.com/rCxcQ.png)


[![enter image description here](https://i.stack.imgur.com/8UyE8.png)](https://i.stack.imgur.com/8UyE8.png)


The photos show a discussion of the addition of an electrophile to the bottom of the vinyl silane. However, what if the electrophile attacks from the top? Why does it still lead to the same product as seen in the photo? I seem to get the opposite product (e.g. E isomer becomes Z isomer instead of remaining as E)




I have used the achiral (*Z*)-vinylsilane **1** as an example with one of the p-orbitals of the double bond colored red. When the electrophile reacts on the "top" of the black p-orbital the carbon rehybridizes from sp2 to sp3 producing the chiral carbocation conformation **2**. The same operation on the "bottom" lobe of the black p-orbital affords carbocation conformation **3**. These two carbocations are mirror images of one another. In these Newman projections the polar silicon-carbon bond (Si$\rightarrow$C) makes a 60o angle with the vacant p-orbital. There is already partial overlap between the Si-C σ-bond and the vacant orbital. Rotation about the central C-C σ-bond of the sp3 carbon by 60o in a clockwise direction in **2** brings the E-substituent proximate to the phenyl group. Cleavage of the silyl group leads to (*Z*)-**4**. The same operation with **3**, but in a counterclockwise direction, also leads to (*Z*)-**4**. There is no overlap between the Si-C bond and the other lobe of the vacant p-orbital.


**Addendum:** There can be no rapid rotation about the central C-C bond in **2** or **3** because that would lead to the same distribution of products as afforded by both (*Z*)- and (*E*)-**1**. If so, the reaction would no longer be stereospecific.  
  

[![enter image description here](https://i.stack.imgur.com/xkjAn.jpg)](https://i.stack.imgur.com/xkjAn.jpg)


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