I'm really curious how the whole dynamic works and I wish the game explained it better, because at this point we can only guess. The distance thing could be true, it could work the same way telepathy works between a true vampire and their bride. He could also be capable of some blood magic manipulation and use these skills on Tav. There is simply not enough information on what the Ascendant can do.
If you haven’t seen it yet, I’d suggest reading through
this thread. Thus far, the vampire bride theory has been the most effective at tying up some of the cryptic explanations Astarion gives and goes the longest way toward explaining his inconsistent behavior post ascension. I especially like the posters discussion of vampire brides/grooms often not knowing the true state of their situation, so as to give the vampire master an upper hand.
