(a nerdy rant) It depends on what the intended purpose is. TENS works depending on the muscle action. So in the flexor, it flexes....but on VERY high settings. Usually you just feel a zing or a wave. To actually get visible flexion or extension, you may need to dial it up to 10. While it feels like nothing or even soothing in a normal body conditions or someone with a right muscle disease.... in a normal, healthy individual, this frequency will be painful.
Now in this instance, (I haven't read the webtoon so all is the speculation) I'm sure they toned it to the max... The person is muscular and in pretty good body condition. To get visible muscle contraction in this body type, you need to have setting at least at 4/5.
Now, even if it is a low-frequency current, it still is a current. And since I'm sure none of these fuckers are remotely trained as a PT or PTA, they dialed it to the max. Which is painful. PAINFUL. Not even "uncomfortable" or "it feels so good". It feels like you are getting electrocuted, but in a single point. Now this person is getting wet... Which is a contradiction for this modality. But nooOooOOoOOoOo.
In short.... No. Its not fun... Yes its worse than uncomfortable....
I'm sorry I kinda went down the rail..... But the more you know...
I'd say it'd mostly depend on the body's condition. Everything in rehab varies from person to person. Cramps are prolonged...they can last from a few seconds to minutes... TENS has wavelength patterns.
So it can be continuous or have breaks.
The amount of current varies and depends on manufacturer of the device.
The device we used for practice in our student days would cause a feeling of shock or electricity running through the body, followed by a very visible contraction. In my own personal experience, my junior changed the modes from muscle stimulation to cutaneous (skin, TENS is transcutaneous electrical nerve stimulation, meaning the nerves across the skin are stimulated).... At maximum intensity. And it was very painful.... And my wrist was in a ballistic movement pattern.
Again, sensitivity of nerves differ ftom person to person, but in healthy tissues, it can get disruptive.
Therefore, I'd say its not fun pain at all. You'd feel tingling even after it turns off and you might even feel pain on simple touch because the nerves are overstimulated.
No cramping, yes.
No pain, no.
Especially if it's on max. Yes there is a jolt of movement but there is not enough time to recover from the shock for the next jolt of movement.
Cramping can occur but it'd require a different mechanism.
The key point is the condition of muscles and the capacity of a person.
Source: I have treated patients with neuromuscular disorders in hospitals and clinics. Even the person with severe muscle rigidity or someone with severe muscle spasticity could barely take 8.
Patients with axontmosis (injury to nerve it the nerve is not cut. Transmission is slow) or neurontmosis (nerve is cut) can bear 10, with no complain.
ALSO DISCLAIMER
PLEASE DO NOT USE TENS OR ANY THING WHICH CIRCULATES CURRENT THROUGH THE BODY, FOR PLEASURE PLAY.
DO NOT USE TENS OR ANY SIMILAR DEVICE WITHOUT PROFESSIONAL SUPERVISION.
DO NOT USE CURRENT IN YOUR LIVES, ESPECIALLY WHEN SEX OR LIQUID IS CONCERNED.
THANK YOU.
Regards: Your friendly neighbourhood Doctor (PT)

The one with this blondie who has TENS patches all over his body (a device that runs low-frequency currents thru the muscles, used in rehab)....IT IS TICKING ME OFF!!!
THAT DEVICE IS SO EXPENSIVE (in my country) AND REQUIRES SO MUCH CARE. WHY TF IS IT BEING USED FOR GANGBANG!??!!
I AM SO MAD (Ik this is a pornhwa but still pissed me off so bad)