Lily Malone alerted me to this article a couple of weeks ago, which I thought fell under the extreme D/s category we're exploring this week. This article is about a male-female relationship where she wears a dog collar and he takes her for walks, with his four-legged dog. The article explains that the collar-wearing is consensual and that they agreed to that before they entered into a relationship. They claim that it's not a sexual act, but just the way they are.
Wearing a sign of 'ownership' is often a part of a D/s relationship, and can be the sign that 'play is on' - a sign that both parties recognise as the commencement of their D/s play (this is especially handy if you aren't in a full time D/s relationship). It doesn't need to be a dog collar as in the article. It can be a piece of jewellery, an item of clothing or the loss of an item of clothing, it can be anything you decide. And no one else may know if the significance, making it all the more exciting and enticing.
I think rape fantasies that many D/s relationships play out fall under this category too. It can be an extreme form of D/s, if taken to a certain level. Although I think rape fantasies can be disassociated with D/s as well...it all depends on you and your fantasy.
The key is that in healthy D/s relationships, no matter how extreme, there is discussion and communication between the parties, and all know and understand what they're becoming involved in, at every step along the way.
Sadism also falls under the extreme D/s fetish (although I guess it is it's own fetish too) - where a submissive can be hurt/injured for the sexual arousal of the Dominant. It's a consensual act, where the submissive has agreed to certain possibilities of the play (e.g. they may agree to whipping but not cutting, or bruising but not in obvious places, they may agree to being tied, bound, gagged, and taken by force, or any other acts the Dominant may desire). This is not something decided on the fly - it's discussed before sexual play, preferably in a non-threatening environment.
I recently read The Saint by Tiffany Reisz. It deals beautifully with an extreme D/s relationship (and a forbidden one) where the Dom is a sadist. He's also in a position of power over the girl, he was her priest, and at no stage in the story did he abuse his power with her. He gave her knowledge and resisted having sex with her until he considered that she had enough experience to be able to 'cope' with what he wanted from her...which was extreme. But it's so powerfully written that although the sex was incredibly rough and confronting, you had to respect the sadist for their care. No matter how much pleasure he took from her injuries, he ensured her safety at every single step.
That's the power of an extreme D/s relationship, I think. You're both free to get to your most basest instincts, knowing the person you're with will accept you, keep you safe, and allow you the freedom to explore your darkest fantasies, or needs.