This is probalbly NOT what you want to hear but if the tutorials don't help, you'd do well to just learn Physics.
If you don't know physics, the real physics, you are fighting an uphill battle with the PHYSICS engine.
Now, if you already know Physics, I dare say the Physics engine is transperent: you apply a non-equilibrium force to an object and this leads to a change of velocity, either an acceleration or decelerations. The constraints are just that, constratnts on the range of motion, velocity, and or acceleration.
If you dont' know physics, read a small physics tutorial on the web (there are a million of them). ONce your done with that, go back to the PE and you'll find it soooo easy.