Most attachment theory centers on the way children form secure bonds with one or more of their caregivers. Later theories extrapolate that pattern to adults and the way in which we interact with peers, etc...

So, if you view your therapist as a caregiver (which some therapist encourage I think) then attachment could be desribed as less anxiety present when the caregiver is absent.

It's almost like knowing that you are okay without them, because they are there, even when they aren't.