Examples of real life movements outside of sports that you use in everyday life are the best examples that you or any other average person can relate to when it comes to the way you do your reps. Try this, Stand up and jump up and down in a slow controled motion. You will find that you can not do it. The use of a little cheating when you do reps with a heavy amount weight is just fine. In fact it is what you are learning when you are lifting big weights.  a big weight training mistake is not using this principle.