The game is progressing, tactics are progressing, analysis is as well. Anyone can beat anyone in a 2 game tie if you set up correctly. You also need to take in consideration the morale part of the sport. It doesn't fecking matter if you're 3-0 down, if you score one and keep up the pressure you're planting the doubt seed in the opponent's head and we know that smells trouble instantly. If you can get on the pressure "high" it's pretty hard for the opponents to respond. When they do respond we get to see an amazing game (City vs Spurs 2nd leg).
Running helps, having attacking players as well, having players that can stop a ball, deliver a cross, pile up the pressure on the opposition, not bottle it when it matters.
Without sounding like an all knowing pirck, it's not fecking rocket science, is it? Only that Man United are bad at it at the moment.
But the basis is never giving up and leaving 110% out there, something that can't be said about half our squad.