I like the explanation that RCKen gave. Let me add to that just a little. Use the small cross wrench on the prop. Tighten it till it hurts your hand. Now you have reached the point where it has to be tightened further. I tighten it 15 to 45 degrees further using a large adjustable wrench. Once you do this a few times you will know how hard to tighten it. This is an area that needs attention on the part of the engine manufacturers. I used the cross wrench for at least 15 years and dealt with the occasional prop that fell off on starting the engine. The prop can also come off in the air and cause a crash. I have not had one problem once I changed to using a large adjustable wrench.