system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: During a shuttle launch, what happens at T-6.6 seconds vs. what happens at 0 seconds?

At T-6.6 seconds, you see a huge fireball come out of the rockets, but the shuttle doesn’t actually lift off until 0 seconds.  What happens at 0 seconds that isn’t happening at T-6.6 seconds?  I think I found a Wikipedia article, but it’s way over my head.
gpt
At T-6.6s they begin firing up the three Space Shuttle Main Engines underneath the Orbiter. These are liquid-fueled engines that pull fuel from the big orange External Tank.  The SSMEs are *extremely* mechanically complex and things could go very wrong very quickly if an engine fails during launch, so NASA lets them run for a few seconds to make sure they’re working properly. 

Once the SSMEs have stabilized, the two Solid Rocket Boosters strapped to the side of the tank are ignited and the Shuttle lifts off. Unlike the liquid-fuel engines, there’s no way to shut the SRBs down after they’re lit, so they wait until the last possible second.