Teams that advance to the Semi-finals are decided in a 2 GAME AGGREGATE SCORE Series.
Chelsea is now ahead 4-2 and the 2nd game will be played in Munich on Tuesday, April 12. Bayern Munich will need to outscore Chelsea by at least 2 goals to have a chance of advancing to the Semi-finals. If Bayern wins 2-0 at home the aggregate score would be 4-4 (TIE) but Bayern Munich would advance on "Away Goals" since they scored twice at Chelsea and Chelsea failed to score in Munich. The same thing applies if Munich win at home by a score of 3-1.
AC Milan will advance unless they lose by more than 2 goals in their 2nd game of the series (next Tuesday) AND fail to score at Inter Milan.
Away goals are a CRUCIAL element to the format of this championship. Juventus, for instance, lost 2-1 at Liverpool yesterday but it can be said that they are still in pretty good shape to advance. All Juventus has to do is win 1-0 at home next week and they will advance over Liverpool. Even though the combined score will be 2-2, Juventus will advance because they scored an "Away Goal" at Liverpool and Liverpool failed to score an "Away Goal" at Juventus.
I hope this clears things up.
MM