35.6Voice leading tells you why the surprise can still work

A7:

A C♯ E G.

To B♭maj7:

B♭ D F A.

Possible movements:

A → A/B♭

C♯ → D

E → F

G → F/A.

Nearly everything can move smoothly.

So:

function may be surprising while voice leading remains convincing.

That is an important distinction.