I don't even know WHY I start writing these things when I KNOW I'm tired and won't make any sense, but let's do this anyway!
Keep in mind that this is a headcanon on how the seal works. It is therefore NOT CANON and there will be no bashing here!
The seal is all in all a leash. I would want to say 'nothing more, nothing less', but merely calling it a 'leash' would be a grave simplification of it.
It's based on alchemy and dark magic since using holy elements would pernanently damage the vampire in an undesired way, and it connects the vampire's mind with the master's.
Through the seal, it's possible to control the restrictions of Alucard's power to ensure that he won't go overboard (which he has a tendency to do). The seal connects them but it's NOT a constant weight or a nuisance physically or mentally for any of the 'participants', so to speak.
It's a leash. Simple as that. Not a rock, not a noose, not a cliff, nothing like that.
Pain and pleasure can both be recieved through the seal depending on what the master does with it mentally. By pushing down on it, it causes immense agony for the vampire, and by tuggin it upwards, it's possible to almost blind the vampire with arousal and pleasure.
Both tugging and pushing have their limits of course, but they're almost impossible to reach. The vampire would faint in pain before the ultimate level of pain would be released, and the vampire would have cum until he passed out before the limit of the pleasure would be reached.
It isn't possible to hear thoughts simply because of the leash. That ability is fully Dracula's own, but it IS possible to feel emotions through the seal. Happiness, calmity, hate, rage, all of them can be felt through the seal, but no direct thoughts and words.
Abraham designed the seal to last for a century. Once that century had passed, it would have to be done again to ensure that the vampire would be bound.
If the Hellsing dies before there's a possible heir within these 100 years, the vampire's mind would die along with the human's. Once the human would die, the vampire would become nothing more than a ghoul.
He CAN be released from the servitude however without causing himself any damage and if a Hellsing without an heir dies after the sealed century, his mind will be unaffected as well.
SO THAT IS THE SEAL EVERYBODY!
Just a little something for you before I go to sleep~
I had planned on writing my headcanon for 'Vampires and their fledgelings' tonight, but I'm too tired.......