This is something that is quite difficult to quantify.

The best description I can give is that it is something akin to a trance state, where the submissive is so far gone into the scene they sort of space out.

Some say that when in sub-space they no longer feel pain. This, obviously, is dangerous if you are involved in impact play, fire play, or the like. For others, sub-space is a mental place where all instructions are obeyed immediately and vigorously without conscious thought.

Ultimately, it is a different experience for each person. As to how to reach it? Very difficult to say. Some reach their sub-space fast, others with difficulty, if indeed at all. Possibly meditative and/or autohypnotic techniques would be of assistance, however.