Now that's the first episode, that plays with more supernatural elements than usual in Watership Down (I simply call Fiver's visions also supernatural

)
Fiver and Silverweed are soulmates and both have a mind connection, that allows them to see through the other's eyes and even control him.
Why not? I like the idea and it adds some interesting tactical, let's call it gimmickry to this episode. Unfortunately, we could only say to this episode, because Silverweed already was freed here by Watership Down. Maybe, the creators could have expanded those "mind games" over several episodes.
But what's really cool imo is the dream, Silverweed keeps Fiver trapped in
