Same as the usual mate.
If the tie is all squared up after 2 games, the team with the most away goals win.
So Bayern cannot go to PSG and win 1 nil - as this will be 3-3 overall, but PSG have 3 away goals. They will either need to win by 2+ goals, unless they win by a scoreline of +1 when over 7 goals are scored (4-3 win to Bayern in Paris will see them progress).
A 3-2 in the Germans club favour will see the tie go to ET.
Me personally - I would like to see the rule changed. I think too much can happen that can swing a tie in favour to one side. I think the away goals rule should be capped at 1 goal. So if any tie ends at 2 apiece, did both teams score at least 1 away goal? If yes, then Extra time.
The reason I say this is because the argument is that the home team of the second tie gets the advantage of having the crowd, home field advantage. However, if it goes to extra time and a goal is scored by the away team - then the home team will need TWO goals to win the tie. This is something that really irks me.