Definition of upswing
In poker, when we talk about a upswing, we're talking about when you're on a winning streak. Let's take a player who makes, in the long run, 5bb/100 hands. This is his hourly tau. In the long run, he will consistently win 5big blinds per 100 hands. On the other hand, in the short term, there are months when he will make 1 big blind per 100 hands, other months when he will lose 2 big blinds per 100 hands, and months when he will make 10 big blinds per 100 hands. This is the variance. The fluctuations are called the swings. Some swings are down (downswing) and some swings are up (upswing).
Don't be distracted by the swings
It's easier said than done, but whether your results are good or bad, you need to keep the right attitude. In downswing, some players will become very fearful. In upswings, some players will be much too aggressive and will tend to bluff more than the optimal way. Try to play the same poker game over and over again; don't stray from your winning style in both downswings and upswings.