I know my boy finds it deeper when he's restrained, preferably heavily. He feels like he can "let go" a bit better.
I'm actually the opposite, as a dominant I find it deeper when they aren't restrained, but aren't allowed to move. This is going to come out wrong, please know that I'm not denigrating anyone who prefers restraints, but it feels "more submissive" when they have a choice, and choose to obey, rather than not having the choice to disobey at all.
As a sub I get that completely.
If you are restrained and you get off on the pain that's great, but if you have to take it unrestrained that is really being submissive to me because you have to overcome your self-preservation instinct (which of course can be a problem in itself).
I have to confess though that it's not the pain that does it for me anyway - its the other person's enjoyment in doing what they want to do, hurting me in the process and it happening whether I "want" it to or not, which is probably the reason for my answer.
For that reason I prefer someone really knowing me and agreeing limits that are to do with outcomes (like damage or the lack of it) rather than using safewords.
Before I start a storm, yes I do understand the risks involved and all of the problems with consent etc. that leads to. It certainly also requires a level of trust, proper negotiation and with faith in someone's abilities which is not always easy to find. The pay off to all the effort only really comes if someone gets off on being Dominant rather than toppy, and you are genuinely responding to something they want to do rather than something you have made up in your head.
That is sometimes not as straightforward as it sounds.