The current generation of transponder control heads use push-buttons to enter the code, so the code does not 'change' until the fourth and final digit is entered. The older rotary controls would of course scroll through the digits as they were changed, so if you paused mid-entry, you'd get an undesired code - some of the more recent rotary units have a few seconds 'pause' built into them, such that the code isn't 'set' until you stop turning the controls for 3-5 seconds (yet again negating the need to select STBY).