To turn your back into your opponent while his arm is free is always a bad idea, and should be avoided as much as possible. In the following clip, when he applies a "shoulder throw", his opponent's left arm are free.
http://www.youtube.com/watch?v=Vs2vKMN4HpA
- His free arm can wrap your waist, spin with you, and drag you all the way down to the ground.
- His free arm can hook punch on your head.
- He can even use his free hand to pull your head back.
Attachment 8858
I had pointed this out in a Judo discussion and make many Judo guys unhappy about it. They all believe that if you spin fast, your opponent won't have time to do anything to you. The problem is what if your opponent is also fast.
However, when there is a problem, there is always a solution for it. If you use "under hook" to lift your opponent's shoulder when you turn your back into him, since his free arm will be lifted by your "under hook", he can't use his arm to wrap your waist. You will be quite safe even you turn you back into your opponent.