I read a post by Brigit Writes about her experiences with pain. It is a very good post so give it a read.
Subspace is a state of mind associated with BDSM activity. It is a pleasant state of mind that submissives love to experience. It is described as floating or feeling detached. A submissive can enter into subspace as a result of the sensations they get from their Dominant. It is a sense of intense focus, and a submissive can lose awareness of everything going on around them.