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.
