The fan motor on our cars has two seperate windings. One set of windings is for low speed. The other set is for high speed.
When both windings are energized, the current draw is very high, and will lead to premature failure.
So you don't want both energized at the same time.
What was described for you was how to test them. If you get a fan with the A/C on, then you are getting the low speed fan. If you want to test the high speed, simply ground pin 17 and turn the key on. You should get high speed fan.
For the most part, the temp setting for the High Speed fan is set so high, that you would only see it turn on automatically in the most severe heat. That is why you might see a diagram mentioned in some posts about a different way to wire the fans to have them come on a little lower. We know that heat is bad for our cars so people have prefered to turn the fans on at lower, more sane, temperatures.