dig_disp++; // variable is increamented
dig_disp=dig_disp%4; // as u can see this line using modulo any value
switch(dig_disp)
dig_disp=dig_disp%4; // as u can see this line is using modulo any value of this variable will be check by the switch case..
this is simply getting the timing if you want faster timing you can change that in 2 instead of 4..
try to experiment so that will know what happens if you change that line..