Since you have no food to push any waste matter out of your system during the MC, you need to help it along or you will feel sick. If you don't you will be working hard to break up and remove all the toxic gunk in your system only to reabsorb it. The sienna tea forces your body to expel the waste. Some people skip the tea and only use the Salt Water Flush each morning but the recommended dosing (per say) is Sienna tea at night to get waste moving and SWF (Salt-Water-Flush) in the morning to wash it out.
If the tea is too strong for you, you can steep it for less time. Even though it seems like this would mess up your digestive system when you are off the cleanse, it doesn't as long as you stop the tea/SWF on your last cleansing day. Then your system picks up where it left off. This is my sixth fast and I've never had a problem when I go back to food.
I suggest you read the booklet (you can find it online) before starting your cleanse. It explains this much better than I can.