what is the short term trading strategy when you watch the two lines?

You could be talking about a number of different technical trading techniques. My guess is that you are talking about the 20 – 80 Stochastic Crossover. Here there are two lines, the fast and slow moving, smoothed lines. When the fast line crosses over the slow line and is crossing over the 20 line, this is a buy signal. When the slow line crosses over the fast line above the 80 mark, this is a sell signal.

I am most positive that this is the signal you are trying to refer to.