Exploring submission play can involve intense sensation. As a normal part of some submissive's play is sub space. Not everyone reaches subspace each and every time and you can not expect to reach it at all. Sub space is a way your body responds to endorphins. Described as similar to a runner's high this is a good feeling and one to be enjoyed if you ever get there.
Subspace is a mental and physical response to the high levels of endorphins produced during play. It can manifest in many different ways and no one subspace is the same as another. Submissives will even describe their subspace completely differently than another person. This is normal and is probably why it's so hard to describe exactly what sub space is in more specific terms. For some people, it is a sense of floating or out of body experience. For others, subspace is a quiet zen-like state. For me, it's when pain becomes funny. I start laughing and giggling. I can take more pain then and ultimately just feel great. I can go non-verbal near the end as well. All grand.
Entering sub space when you are a novice can be dangerous. When in this space your ability to stop play or fight off anyone is gone. You are at the mercy of the Dominant. If you do not know this person well you could be setting yourself up for harm. Always negotiate all play, including the level of space you wish to get to. Keep talking if you want to stay out of space, stay engaged in what is happening and you will be safer.
Sometimes in space, you can not feel pain the same way and so you can take higher levels of pain. The danger here is a Dominant that does not know you well could hit you too hard and you could end up with damaged nerves or broken bones. Trust is important in a relationship so keep space to a relationship unless you have the trust with a friendship Dominant.
There are essays all over the internet that describe different forms of sub space. None of them are wrong. and you can learn from all of them. One of my favorites is Mistress Steel's description of all the different spaces that people can go through. You can read the essay at The Steele Door. She breaks down sub space into several levels with descriptions of each level. I like how it's described enough that you can see yourself in the descriptions and follow how you ebb and flow through your own space.
How would you describe your sub space? Do you need pain to get there?