I have an E-WMCBP (Electrified World's Most Comfortable Butt Plug) with a diameter of 1.75 inches (4.5 cm) and a usable insertable length of 3.5 inches (9 cm). It is fairly weighty at 14 ounces (0.4 kg). Not the biggest plug I have or the heaviest. But it is, indeed, quite comfortable. Made out of aluminum with an insulator between the base and the body of the plug, it is a bipolar plug. One of the 4mm banana plugs is connected to the made body of the butt plug and the other is connected to the base. This, of course, is plugged into a power control box. I have a TENS 7000 for "lighter" portable play. For this anal plug, I use an E-Stim 2B box (powered by a 9V battery or a separate universal plug-in power adapter. With the plug in adaptor it has about 30 more power). A more elaborate selection of power control modes than a TENS unit and at higher power.
There are two power levels to all the wave forms (Low and High). There are too many patterns to describe here, so I'll pick only two: continuous and pulsed and will concentrate only on the butt plug action. I have other options but the original question is what does it feel like?
Let me start with the basics. The power box converts a DC source of electrical power (e.g., a battery) to an AC power source. It is a simulated AC wave form that mimics the AC wave form that you get through your wall sockets. That wave form is produced by a rotating magnet passing within a set of wire coils to induce an AC voltage and current that has a sinusoidal wave form at 60Hz (North America) or 50 Hz (pretty much the rest of the world). So, what you get and feel with an electrostimulation system some waveform that is based upon AC voltage and current. It's much more complicated than that. But it gives you the basic idea of the type of power being applied. Voltage can be though of as "pressure" to move the electrons along a (resistive) path and current is the number of electrons that are flowing along that path.
With the base tucked tightly against my anus, the current flow (it's AC voltage at less than 55 volts) is between the body of the butt plug and the base and the current will flow along the path(s) of least resistance between those two points to cumplete the electrical circuit. This means the pathway for the current flow is not always constant. The current flows through the muscle tissues of my rectum and the anal sphincter muscles. Each person will be different and their experiences will also feel different at different setting levels.
So, let me describe what the pulse setting feels like to me. I can set the frequency of the pulse (minimum about 3 times per every two seconds) to a very fast pulse approaching continuously. I can also change the "spikiness" of the electrical wave form from something that simulates a sine wave to something that is quite spikey. Because the contact area inside the rectum is so large, the change in the waveform shape is more subtle. The change in frequency is not. And the slower the pulse, the more powerful each pulse feels at any given intensity level. What each pulse is causing is the muscles in the rectum and around the anal sphincter muscle ring to contract. The contraction of the muscles with each pulse feels like the butt plug moves (and it does slightly) with each pulse. But the sensation of the degree of movement is greater than the actual movement. It is not, for example, enough to draw the butt plug in. It does, however, feel like the butt plug is thrusting in and out. Depending upon the voltage and current applied, that sensation can be mild to quite intense.
I had seen the term "ghost fucking" referenced with regard to electrified butt plugs and the application of a pulsed signal. It is not far off as a description. It can feel like an anal stroking even though there is not a physically long stroke taking place. It can be quite pleasant, too, at lower intensities. I've fallen asleep on my side with the butt plug in, and the pulser power at 70-75% of the power I might use for stimulation towards orgasm. It does disrupt sleep patterns (less REM sleep). Butt to wake up to the sensation that there is still a pulsing in your rectum can be quite pleasant and very different from "just wearing a butt plug to bed.
Most programmed patterns are some version of a pulsed electrical signal being sent. The direct contrast to this is a continuous electrical signal being applied. By continuous, I should note that the wave form is an AC one and not a DC (distinct positive and negative polarity like a battery). But it also means that at any setting, it has the equivalent of a non-varying AC voltage being applied. The current flow might change because the "resistance" to flow might change by how and along what path the electrons flow. Rather than feeling pulses, it's more like a continuous "clenching" of the muscles of the rectal muscles and anus muscles. As the power level increases, this sense of clenching also increases dramatically. And at very high levels, it can feel like the butt plug is moving (and it might be only slightly) as the pathway for current flow changes to follow the path(s) of least resistance to current flow. Again, I can fall asleep with a low level continuous voltage and current flow with the butt plug securely "clenched: in my ass. I haven't found it possible to fall asleep with the power levels turned up significantly. Waking up in the morning with a continuous lower power applied overnight is like suddenly relaxing you ass from being forced involuntarily to hold something in all night. It's not really noticeable until the power is turned off and the muscles "relax" even though the butt plug is still in place.