Sephiroth will tell Cloud to shut off the valve outside, which is done by holding L2 and R2 multiple times. With that done, re-enter the room and a series of cutscenes will commence. Down here ...

You’ll then go to Nibelheim, where Cloud can visit his mom ... where ya gotta hold down L2/R2 while people stand around Sephiroth holding and never using their guns (you and I both know that ...