Guys, I am having an issue getting the engines started.  With the APU running, and as far as I can tell all switches configured correctly, when I go to ground on the start switch they will spool up but will not light off.  Reading another post I saw someone mention hitting CTRL-SHFT-F4 before going to ground on the start switch, so I tried that and it worked.  My question is, what is that command doing and why do I have to do it to get the engines to light off?