Mass protests and voter activation might have tipped the scale towards Lula. There was a rude awakening among mainly the younger populace when Bolsonaro jr managed to get the most votes in the first round.

What does "voter activation" mean in a country that requires that every person who can vote must vote?

~3 million more people voted compared to the first round. 30 million still didn’t vote. You just pay a fine to put your registration in order for the next time, there are no long term consequences.

Just because you can vote doesn't mean you have to vote on the first candidate or the other one, voting blank or voting arbitrary is genuine options.

So voting activation is to push all those people to not vote blank or arbitrarily.

Means +3M voters on the second round. While voting is mandatory no one is forced to vote