Penetration is one of the most common aspects of BDSM. It is also one of the least recognised and understood.
It's likely that if I were to mention the word "penetration" to a BDSM person they'd first think that I was talking about inserting things---animal, vegetable, or mineral---into one of the bodily orifices located somewhere between the waist and the knees. In other words, they'd think I was talking about sex.
In male/female sex it is the man who penetrates the woman with his penis entering her vagina. But penetrate can also mean "To affect or influence deeply", and this is the meaning which is most important in BDSM.
We can see this form of penetration in action in, for example, flogging scenes where one person, the top, lays into the other person, the bottom, using a flogger. The bottom feels the thudding and the pain from each stroke. Similarly, and literally, we can see penetration occurring in cutting or piercing scenes where the top uses a knife or hypodermic needle tips to break through the outer layer of skin of the bottom. And, in psychological play we can see a form of penetration occurring in activities such as humiliation, where the top uses words and degrading behaviour to break through the defences of the bottom and affect them.
If it were only the physical sensations or feelings themselves which were important to a bottom or submissive, then we would discover that much of BDSM could be satisfyingly performed solo with merely the aid of flogging or whipping machines, or with CD players (for humiliation). But this isn't the case in any part of the BDSM world.
In fact, what we see in such scenes is a heavy interaction between the participants in terms of touching, caressing, talking, murmuring, flogging, cutting, or piercing; and other communication of mood and feeling by means of sounds (moans, groans, etc.) and movements (writhing, twisting, relaxing, tensing up, etc.).
Thus, it is the case that the reactions of the bottom to the ministrations of the top also penetrate the top at the same time that the actions of the top with the flogger, blades, needles or words (and the touches, caresses, murmurings, etc.) penetrate the bottom.
This penetration of the top by the reactions of the bottom at the same time the bottom is being penetrated by the top with his actions is key to understanding what the top gets out of the activity. If it were merely one way then, similar to the bottoms-doing-it-alone-with-a-machine suggestion above, we'd find tops alone in their dungeons or bedrooms flailing away at large cushions, or sticking needles into inanimate objects, completely satisfied and happy with the result.
That's not the way it is, and this two-way street of penetrating and---at the same time---being penetrated, is a vital, necessary, and defining part of BDSM.
The following is an extract from Understanding BDSM Relationships by Peter Masters, p. 18:
Passive versus active
One may be tempted to think that in the average, garden-variety scene that the top is the active partner in terms of controlling penetration, but this is not the case.
While the top may be in charge of what happens in terms of the main activity in which he and his bottom partner are engaged (such as cutting, humiliation, etc.), it may in fact be that the top is being penetrated more by his bottom than the bottom is being penetrated by him. This is because a particularly expressive bottom may be sending more the top's way than the top is actually giving to the bottom.
For an activity which requires a lot of focus from the top (such as cutting), this can easily be the case, because the top may only be administering very localised sensation, which the whole body of the bottom may be visibly (and audibly) responding back to the top, feeding (if you like) the top's own need to be penetrated.