For those (unfortunate souls...) not perfectly acquainted with...

For those (unfortunate souls...) not perfectly acquainted with the meaning of the word "sphincter", and how, and why our good mate YAK used it, here is the meaning.....
Or 99% of it.....Sphincter=Ring or muscle surrounding and serving to guard(???) an opening.....
Now.....I left 1% and an opening unanswered......Use your NOUS!!!!!!!!!!!!!!!!!
Kudos gallore for those superlative paradigms of intelligence that will come up with the right answer....!!