Sometimes when I jump into a blowing fan in Super Meat Boy, I fly into the air. Other times I do the same jump, land on the fan and die immediately. I can't figure out a consistent model for how the fans work and how I'm supposed to use them.
It's just physics.
If you land on a fan from higher up, or with bigger downward velocity, you will be able to get closer to it. If you come too close to it, you die. Otherwise, there will always be a time when your vertical velocity goes over zero. And the closer you are to the fan in that moment, the more time the fan will have to propel you upwards, and the higher you will be able to go from it.
So basically it's about finding the balance. Usually you should try to get as close to the fan as possible without dying, and for that you need to find an appropriate downward speed.