Minecraft wiki entry for Advanced_redstone_circuits, associated text: Now we have to make sure, that the state will be erased, if the following digit is wrong. Therefore, we handle a key-press-event (--/b1 OR b2 OR b3 OR b4\--/dt-\--/dt-\--). Search the diagram for the three blocks near "dt-". Here we look, if any key is pressed, and we forward the event with a minor delay. For resetting /A\, if the second digit is wrong, we combine (key pressed) & (not B). It means: any key is pressed and the second digit of the key is entered false. Therewith /A\ will be not reset, if we enter the first digit, /A\ only should be reset, if /A\ is already active. So we combine (B* & A) =: (AB*). /AB*\ now resets the memory-cell /A\, if the second digit is entered false and the first key has been already entered. The major delay /dt+\ must be used, because /A\ resets itself, if we press the digit-button too long. To prevent this failure for a little bit, we use the delay /dt+\. The OR after /AB*\ is used, for manually resetting, i.e. by a pressure plate.