Bottom space refers to the mental shift when submitting to a partner during BDSM. "Entering bottom space" is making the necessary adaptions in the bottom's mind before they can really submit to their top.
This is usually achieved by means of ego reduction and works in a similar way real world prisoners or slaves are and were put in their place by their warders or owners.
The shift can be induced through manipulating the appearance of the sub. A popular and effective initial procedure is seizing their footwear, thus making the sub go barefoot. Further taking away their underwear, mounting up to forced nudity can be used as well as a slave collar.
A further approach lies in restraining the sub, typically with handcuffs and shackles or simple rope. The jangling sound of chains and the sensation of cold metal on the bare skin typically brings along an uneasy feeling for the sub already. The most common way is to start with cuffing or binding their hands behind their back to easily induce an acute feeling of helplessness.
Common forms of verbal humiliation and rituals like standing at attention, assuming degrading positions, getting called by a slave name or prisoner number, while having to call the top "Sir" or "Madam" (Ma'am), "Master" or "Mistress" often complete the transition. For a prison roleplay the correct response to the top would be "Yes, Mr. Overseer, Sir!" or respectively "Yes, Ms. Overseer, Ma'am!".