Rule 4. If your current step results in more than doubling, then you must spend the surplus on yourself and leave only the approved stake money. For example, if you are on step 17 and manage to change your stake of £320 into £700, then you must spend the extra £60 (leaving you with £640) and not carry this onto the next step to make it easier. Put simply - start each step with only the stake money allowed for that step.
Rule 4. Detail. For example, you do better than expected at step 14. Instead of £80 you find yourself with £90. This rule means that you must retain £80 and spend £10 on yourself as a treat. You must not use the full £90 as stake money in step 15, to make this step easier.