I do this all the time and have all my life since I have been DID since childhood. I go to the dentest and end up getting about 5 different shots to numb the work area. For example -

I go in sit down get shot #i
the drilling triggers a switch into for example Mary. Because the memory content of Mary doesn't have anything to do with getting that first shot so I as mary once again can feel the drilling. in goes shot number 2.
During the work on my teeth I end up switching yet again and because memory piece number three does not contain and content of getting shots 1 and 2 I can as memory piece number three feel the work being done so in goes shot number 3 and so on and so on.

Then I leave the dentist once again the first person and that medication from shot number 1 wears off but because as other memory pieces I recieved more medication each time I switch into those I am numb for about 15 minutes

The way it was explained to me is that each piece of memory contains the vitals - heart beat, breathing rate, body chemical make up of that piece of memory that was separated.

For example switch into the memory piece of a memory of being 5 years old the brain will automatically change the heartbeat, breathing rate, body chemicals and so on to that of what is contained in that memory.

for example if the original situation was that I had to perform oral sex and I gagged and had trouble breathing and I was anemic and had a food allergy to tomatoes.

In the present when switching into that memory piece the brain keeps my body functioning by automatically acting out that memory content - so as a 40 plus year old switched into this 5 year old memory piece I would be gagging as if I was still performing oral sex, gasping for air and if someone took my bloodwork I would show up anemic and alergy testing would show an allergy to tomatoes.

The brain automatically switches to what is contained with in the memory piece that is stored at the unconscious level that got triggered and keeps the body funtioning on that memory content.

When I switch back the brain automatically stops keeping the body funtioning on the unconscious memorys but instead on the present consciously stored memories - since I am mentally connected to what is going on in the present.

The brain automatically returns the body to the last rates and chemical balences that the person was experiencing right before the switching because that is the conscious memory that is stored.

Which is also why switching back to conscious awareness causes headaches - the body has basically went from the body chemicals, heart rate, breathing rate of a 40 plus year old to that of the 5 year old memory piece and then back to the 40 year old body chemicals, heart rate, breathing rate and so on.

if being drunk was the last conscious memory stored then its drunk that you return to when coming back into conscious awareness.

one time I had the flu I remember being in the school nurses office and being sent home with the flu and then suddenly it was three days later and I felt crappy and my parents didn't believe me they said I hadn't been sick for three days and to knock off the faking I was going to school. Got on the bus and threw up and got sent home.

Its just the way DID works.