Understanding how upsets occur can help bring them off in your own games. They don’t come about in the way that most people think, for example trying a quick attack is unlikely to work.
The most valuable weapon an underdog has at his disposal is in fact frustration. If a higher rated player is having trouble beating you the chance of an error increases massively.
Here’s a case of this from the recent Blackpool congress in which GM Mark Hebden would have desperately wanted to win. So desperately in fact that he blunders with 47.Ke7?? after which 47…h4 48.Be6 b5 as White can’t cope with the passed pawns on both sides.
Instead 47.Bc4 doesn’t have this problem as 47…h4 48.Bf1 b5 49.Bxb5 h3 50.Bf1 h2 51.Bg2 still stops the pawn. And other moves such as 47.Bc6 and 47.Kf7 also draw.