Yes, you're right. I meant bouncing. For example if I take a 5V signal and put it (physically) into pin2 on the Uno it enters the interrupt function. However, that change to 5V isn't a smooth transition and bounces a while before settling into 5V(Which would cause the program to enter the interrupt function several times instead of only once). My particular program is quite long and putting it into the main loop would defeat the entire purpose of the interrupt, since the interrupt can occur at any moment. The if statement is there because the output pin has to change from low to high if the interrupt pin measures high. The same applies for when it measures LOW.