What is a strap-on?  It’s a dildo, but then you know that silly boy!  It’s a very special dildo because it is most often intended to be ‘used’ by Dominant Women on submissive subjects of both sexes.  In either event, the spot where “the rubber meets the road” (pardon the pun!) is the anal or vaginal orifice of the submissive.
Most commonly, strap-ons can be obtained in both latex or silicone rubber, and both versions feel and perform their task just about the same.  Personally, I prefer latex over silicone because the former substance is impermeable to bodily fluids whereas the latter is not.
This feature makes a latex toy easier to clean and disinfect afterwards.
To explain the toy’s design, it may be best to start by thinking about how it does its magic.
It sounds simple enough:
The rubber shaft shaft forms a key part of a pelvic engine, which is comprised of the pelvis of the Dominant Woman and the toy itself.
Like a piston, the shaft moves back and forth, in and out of the body cavity, in the process stimulating through pressure and by feel the nerves lining the tissues through which it passes.
To the submissive, this is where the magic occurs:  the passage of the piston produces ecstasy and joy — and sometimes a little pain!
You can readily see that, for that stimulus-producing movement to take place, the shaft must be alternately shoved and pulled, mimicking the movement of a penis, and these actions, in turn, dictate the design of the instrument.
The flexible rubber shaft is attached to a flat trapezoidal piece of leather or very stiff rubber, perhaps three inches wide and four inches high, that rests snugly against the pubis bone of the Dominant Woman who supplies the energy to power — really, to drive — the shaft home.
This pubic resting place provides the ideal platform from which to transmit Her pelvic thrust down through the shaft when She rams it home.
Simultaneously, the rubber trapezoid serves to broaden the tender area that must absorb the pressure of Her thrusts — Her vaginal muscles lack the solid mass to perform this job well.
Okay, this explains the shoving.  But what about the pulling?  Three holes in the trapezoid allow accommodate rubber straps that wind around each of Her legs and Her waist, the straps functioning like natural ligaments.
Not only do the straps keep the harness snug against Her pubis, but when She pulls back, the straps and trapezoid together act as a harness that pulls the shaft from its orifice.
To use a crude mechanical analogy, the pelvic engine is half bulldozer and half tractor, the bulldozer shoving — or more accurately, thrusting — the shaft home, and the tractor pulling — tugging — it back into position for the next thrust.
Am I turning you on with this description and the visualization, My pet?  Have you ever had a dominant woman use a strap on – on you?