Like a strange odometer (-128 to +127)

To form a crude analogy, the signed char type is sort of like a strange odometer in a new (and unusual) car that shows a mileage value of -128 when you first purchase the car.

As you drive the car, the negative values shown on the odometer increment toward zero and then pass zero.

Beyond that point they increment up toward the value of +127.  When the value goes beyond 127, it starts over at -128.