I see a lot of MICR where I work. Basically, the machine reads each character as a series of up and down spikes, and the number, timing, and amplitude of the peaks tells the machine which character it’s reading.
As a check moves past the magnetic head, the head sees a changing voltage. The head is perpendicular to the direction of motion, so all that matters is the amount of ink in a vertical line intersecting each part of the character. This means that with some Photoshop work, you can make your own MICR-like characters that don’t resemble digits, but appear exactly the same to the mag head.
When the amount of ink in front of the head increases, the voltage spikes up; when it decreases, the voltage spikes down; and when the amount is constant, the voltage is flat. It’s easy to recognize characters if you watch the mag head’s output on an oscilloscope.
For example, the MICR 0 is essentially a thin square. There’s a sharp peak at the left vertical bar, then an immediate drop and a low voltage in the middle of the character, then another sharp rise and fall at the right vertical bar. The MICR 8 looks a little different, because there are smaller vertical bars next to the main “legs”; the head (reading left to right) sees a small rise, another rise, a drop, a sharp rise, a small drop, and another drop.
Notice that MICR 2 and 5 appear almost exactly the same to the mag head: a small rise, a drop, a low plateau, another small rise, and another drop. The only difference is that 5 is a little wider, so the plateau is longer; if the check reader’s motor speed is a little bit off, it can confuse the two digits.