Denki Furo, which translates as Electric Bath, is a fixture occasionally present in Japanese public baths known as Sento or Onsen. There is nothing hidden in the name. Denki Furo is exactly as implied - a bath with electricity flowing through it.
Ultimately it looks like any other bath in a Sento. It is a steel tub filled with very hot water. Across the calm surface (it is rarely occupied) you can see small standing waves ripple and scatter. These are caused by two electrical plates, mounted on either side and submerged in the water. These are what induce the "mild" current. The tub is typically small, big enough for one person only, and the water is heated like other Japanese baths, up to 45 degrees Celsius.
You're probably wondering, what the hell? Why does that exist? And will it kill me? The best answer I can give to all these questions is dubiously I don't know. There is very little information online for westerners about Denki Furo. This is what makes them so scary to try. All you know is that it may kill you, because according to a large sign above it, it shouldn't be used by anyone with epilepsy, Alzheimer's, pregnancy, any kind of heart condition, or anything else.
As you slowly lower yourself into a Denki Furo your limbs are immediately hit with an intense sensation of pins and needles. It is as if the flesh inside your body were rapidly expanding and contracting in tiny spheres all over. As you begin to lower the rest of your torso into the water the sensation increases. Now your whole body is being shaken violently from the inside. Electrical impulses shoot up your spine, from your tailbone, and across your hips. All your muscles twitch and spasm, tensing and releasing. It isn't completely painless, with twinges here and there, but nor it is so unpleasant.
The effect of the bath is much stronger nearer the plates on the walls. The middle of the bath is somewhat mild allowing you to place your torso there without too much unpleasantness. You can dangle your feet closer to the plates but putting your torso up against will really make you feel the electric in your vital organs; something that isn't nice. Putting your limbs near the plates can numb them to a point of mild paralysis. As if they have fallen asleep, the muscles will contract and remained fixed, unable to move.
At this point you better try to use your other limbs to recover the ones lost, lest you be stuck there forever, and slowly sink to your death. This bath is not for the faint of heart (quite literally).
The real healing powers of the Denki Furo are almost certainly rubbish. But that isn't what you go for. You go for finding out what it's like to be electrocuted. Thanks Japan.