Why is van helsing still able to tune in on dracula's movements through hypnosis of mina?

Dracula bit Mina and drank of her blood. Subsequently, he forced Mina to drink of his own blood. That created a connection between the two of them. Due to that, Mina is no longer pure, which is shown in the scene where the communion host burns her skin when it touches her forehead. Van Helsing is able to make use of such connection by hypnotizing Mina and using what she sees and hears to determine Dracula's movements.

moon-letters were a method of writing invented by the dwarves. moon-letters were ordinary runes, but written in such a way that they could only be read when the light of the moon shone behind the writing. some moon-letters could be read by any moonlight, but it was possible to make the writing even more secret. more sophisticated moon-letters could only be read by the light of the same shape of moon, or at the same time of year, as when they were written. hope this !

do you know the answer?