Subspace isn't something all subs achieve or even desire, it generally refers to state of mind where they completely disassociate from reality and won't even think to use such things as safe words or warn the Dom/me if they, the sub, are in over their heads. This means that unless you're an experienced Dom/me you're better off not trying for it to start but working towards it through building up the trust and rapport between you and your sub is built up and you get a better handle on when they've reached their maximum safe limits.