Yep, it will drain it through the skin.
It's not laundry detergent, though. That will probably just make you clean and smell fresh like a forest. It's a different thing, I think originally designed to soften hard water or something... Here in Australia, it's kind of hard to get, but my coach and I (as well as the masseur who was the one to put it on my knee), when we see it, we buy the entire stock from the store!
Here is some info about washing soda:
http://www.care2.com/channels/solutions/home/440