Here is what Cathe says on her FAQs re. STS:
Why does STS have two Active Recovery weeks and what does this mean?
There are two one week active recovery weeks allotted during STS. The first "rest" week will occur at the end of week four which is the end of Mesocycle #1 (muscle endurance). The 2nd "rest" week follows the end of Mesocycle #2 (hypertrophy) which is just after week #9. So your two active recovery rest weeks are week #5 and week #10. By active recovery week we don't mean that you lay around the house doing nothing, but at the same time you need to let your body and mind relax during this week. As Cathe says, "we're taking a break – not a vacation".
One of the hardest things to teach serious exercisers is that by taking breaks they will improve over the long haul more than if they had worked out nonstop. An active rest recovery week will also help to prevent overuse injuries and mind fatigue. During this week you might want to try something like a relaxing Yoga video or a Cathe stretch DVD. Perhaps you will want to do some outdoor activities like walking, tennis or hiking. Whatever you do, it should be a light activity as you're letting your body recover from the previous Mesocycle while getting ready for the more intense Mesocycle to follow.