Act II of Death of a Salesman is quite revealing in many ways. The setting is the reception room in Charley's office, where his son, Bernard, is sitting, whistling to himself, as he waits for his dad whom he has come to visit. 

Meanwhile, Willy is having a major flashback in the elevator going back to the day of Biff's huge high school game at Ebbet's Field. This game is the turning point in Biff's and Willy's lives: the moment where Biff could have made it big in life. According to Willy, this was certainly going to happen. However, here he is, almost twenty years later, going to Charley's office to borrow money. During this particular flashback, a much younger Charley downplays Wlly's extreme excitement about the game. This is why, as Willy comes out of the elevator, Willy is actively re-living his argument with Charley that night. Could it be that Willy's inner-embarrassment is flaring up when he realizes where he stands in life at 63 years old?

To rub salt on the wound, Charley's secretary takes Willy to Bernard, since she does not know what else to do with him. Bernard is described in the play quite differently from Biff and Willy:

Bernard is a quiet, earnest, but self-assured young man.

As they catch up, Willy is amazed to see that Bernard has done so well for himself. He is surprised to know that Bernard is a lawyer, that he has a case pending in D.C., and that he knows people who own their own tennis court in their houses. When Willy tries to embellish Biff in front of Bernard, he finds no other choice but to, sort of, give up. Hence, in an act of complete humbleness, Willy asks Bernard

Willy: What- what's the secret?

Bernard: What secret, Willy?

Willy: How- how did you? Why didn't he ever catch on?

Here is when the most important conversation in the play occurs. Both, Willy and Bernard agree that, after that one Ebbet's Field game, Biff ends up flunking Math, and ruining his chances for college, unless he goes to Summer school. It is then when Biff starts his deep disconnect with the rest of the world. Bernard tells Willy that, after Biff flunked Math in High School, Biff was more than willing to go to Summer school and re-do the class. However, right after Biff visits Willy in New England, everything in Biff's life changes. Willy is shocked to hear this revelation. However, Bernard says:

Well, just that when he came back-I'll never forget this, it always mystifies me [...] (HE) took those sneakers with "University of Virginia" printed on them? He was so proud of those, wore them every day. And he took them down in the cellar, and burned them up in the furnace [...] I've often thought how strange it was, that I knew he'd given up his life...

This is when we find out exactly what happened that day: Biff had gone to New England to vent with his father the fact that he flunked Math, only to realize that his Dad was there with a mistress. The image of "The Willy Loman" that had fed his ego is now, officially, dead. In turn, Biff's own self-perception dies with it as well. After all, he is just a creation of his father's own missing ego. Hence, the encounter with Willy and the mistress is the triggering event that ruins and changes Biff's life, for good.