"Letting go" for me means releasing control to some one else. Not just physically, but in a way, mentally and emotionally as well. Whether it's accessing subspace, rope space, orgasms, or (as I'll touch on in a bit) something much more deeply emotional - it all shares a similar quality.
Because once I give consent, it gets very difficult for me to say the word "no." It becomes way too easy for someone to add to that emotional pain bank that I can withdraw from when I let my emotions top me back.