Hi Q
Welcome to the club!
Three of us at VS tried to work our way through the wiring diagrams for the WAI to make sense of what was happening. The other two guys were much more up to speed on these things than me and eventually could follow what was happening but it certainly ain't easy!
(The logic of it is all too easy, it's the wiring interconnects that are a bit mind-boggling.) Quite obviously, one doesn't want to pump hot air into the atmosphere when the LEs are deployed and there's the run back problem to avoid as well.
The distinction made in the Boeing FM between Auto and Manual is 'inhibited' (for Auto) as against 'ineffective' (for Manual). Pushing hot air straight into the airstream would certainly be 'ineffective'
.