Well, you need 8 before you can have either 9 or 10, you must have a stop level in mind or in place before entry. 9 is to counter the novice (but not just novices') mistake of pushing out the stop as price moves against you.
As for 10 - well, if your car is skidding and you're going to hit a wall, wouldn't you try to scrub some speed off by braking? More specifically, it would not be a bad thing to halve the loss on half the position that is headed towards the stop: and it lessens the psychological barrier of triggering the stop. Sure, the price could rebound before it hits the stop - you should still break eve or make a reduced profit: and maybe the rebound will generate another entry signal, in which case closing the first part of the position has released capital for this new entry.
These rules are mine, they have been published before, by me. They are aimed at eliminating simple mistakes - but although most mistakes are simple, that doesn't stop people making them, and a codified guide like this has helped me with maintaining disciplined thought processes and eliminating emotion / subjectivity.
