For me, it's two things. The start of the session where I'm waiting, on my knees with my tribute and I hear the approach of the Mistress. That feeling of anticipation and bit of anxiety get me every single time in the best way. I get a "I can't believe I'm here and doing this! (Again)" feeling and then when the door opens, reality sets in lol. That feeling is just something else entirely haha.
For the second thing, I have a special love of sensory deprivation at this point, and it's quickly become my favorite experience, even moreso than being feminized. I love how it feels to not know what's happening next or what to expect, bonus points if I'm tied/chained up while sensory deprived! I love how helpless it all feels