Ok, I'll try to explain. Imagine a button with a long distance from "not pressed" to "pressed", like an old Keyboard for example.
Now imagine pressing a button in slow motion. If the metal connectors of the button and the one below the button slowly approach each other, there are moments when the connectors almost "meet" and current flows. One nanosecond later the current isn't there anymore and another nanosecond later its there again. This process continues until youve fully pressed the button down.
But you don't want the letter do appear twice or more! So you've got to make sure that only one letter appears by telling the device, to only print 1 letter in the time, a normal user needs to press a key fully down.
LoL, i hope you can understand what I meant, I can't explain it that good in english