Recall from the first page of this chapter that silicon stabilises carbocations β to it and carbanions α to it. Thus vinylsilanes can undergo both electrophilic substitution and nucleophilic addition.
For maximum stabilisation of the carbocation, the C-Si bond must be in the same plane as the empty p orbital (of the carbocation) as this gives maximum overlap of the C-Si σ bond into that orbital.
Rotation will occur about the C+-C bond to reach this maximum overlap conformation and will avoid having the two orbitals at right angles during the rotation. This is very useful to know when considering the stereochemistry of the outcome.
Irreversible nucleophiles (generally organometallics) are preferred.