What we do to our partners, what they do to us, and the way our relationships play out with them, need to have positive outcomes. They need to satisfy wants or needs. This is our motivation. This is why we do BDSM.
Core to this is the idea of doing and what effect this doing has. For example, piercing someone's nipples may cause them to go into subspace, flogging someone may give them a cathartic release, or collaring someone might allow them to achieve surrender. There are always two parts: what we do, and the effect it has.
What we do BDSM-wise isn't always going to have the same effect though. The same person may react differently depending on circumstances, and other people may or may not react at all. Bondage, for example, may be the bee's knees for some people, but it may be totally uninteresting for others.
When what we do---whether it's a physical or psychological activity, or whether it's a BDSM-flavoured relationship---has an effect on our partner, then we can say it penetrates them. What we do enters them and causes change.
Some activities are almost always going to be on the money in this regard. Many bottoms get pleasure out of bondage or flogging and so by doing these it's hard to go wrong. But doing some activity based on the likelihood that it's going to have an effect is very different to doing the same activity knowing the exact effect it's going to have on this particular partner. In the former case you're just being hopeful. In the latter case you are actually engaging your partner.
Doing something and knowing the effect it's going to have is important for the satisfaction of every BDSM participant, be they top, bottom, dominant, submissive, master, mistress, or slave. Just as much as a top or dominant needs to choose activities which will meet the wants and needs of his submissive, he must also choose activities which allow his own wants and needs to be met. As such, openness and honesty are necessary for engagement and, consequently, satisfaction.
When someone isn't honest, either with their partner or themselves, then the activities they do are not likely to hit the mark. Someone who says they're into BDSM just for kinky sex because they feel uncomfortable admitting, either to themselves or to their partners, that they'd actually really enjoy some rough handling, is going to find that they mainly get lots of kinky sex and perhaps only incidental rough handling when they might actually be far more satisfied with things the other way around.
Thus, the openness and honesty which leads to real engagement is vital for our the meeting of our own, personal wants and needs. Without it, we can find that what we get out of our BDSM is just nearly OK, or almost does the trick, without being 100% right.
At the same time, engagement is important in the context of longer-term relationships:
The following is an extract from Understanding BDSM Relationships by Peter Masters, pp. 20 - 21:
However, just because there’s another person taking part in a BDSM scene with you doesn’t necessarily mean that you have any sort of meaningful BDSM relationship with them. It is important to recognise this difference between simply doing BDSM and being part of a BDSM relationship. Doing BDSM may merely be allowing someone to tie you up and enjoying the embrace of the rope, or can be flogging someone and watching them writhe and see their back turn red. Both of these and other activities may be enjoyable, but don’t necessarily mean that you’re part of any sort of BDSM relationship.
We could say that a BDSM relationship requires an intimate and personal involvement rather than just a technical involvement. Indeed, the idea of engagement means that what we are doing is interacting with our partners in some form or other and that they are interacting with us. Merely hitting them and seeing them jump is not necessarily such an interaction. You could go around hitting all sorts of people and watching them jump but it doesn’t mean that you are having a relationship with them.
Engagement is my third pillar of a BDSM relationship. Going to a BDSM party, meeting some cutie, tying her up, flogging her, and being turned on by it is not engagement. She is not engaging us and we aren’t engaging her. In fact, we really don’t know her from a bar of soap although she may have better breasts. Instead, we are actually engaging ourselves by imagining that she is feeling or thinking in some particular way. We respond to what we imagine she is feeling, not to what she is actually feeling. In other words, while we are performing the physical aspects of our BDSM on her we are actually engaging something conjured up in our own imagination. This something we have in our imagination could perhaps be called the Average Bottom or the Average Submissive. If we do want to play with someone new then we do what we know the Average Bottom likes or what the Average Submissive likes.
The same thing applies to submissives and bottoms who want to play with or impress a top or dominant they fancy. Not knowing this person they do what the Average Top or the Average Dominant likes and hope for the best.
This is an important idea because until we know someone well, until we know what triggers they have, how they feel and respond, and what their own intimate wants and needs are, then we are limited to engaging what we think they want or need and what we think they are feeling, not what they actually want, need, or feel. It may well be that with a lot of experience we can get pretty good at guessing what makes someone else tick, but until we do actually know the real them then we’re still only going to be engaging what we think they are, not what they really are.