When a man massages a woman's labia minora slowly with his tongue. This is said to be a "turtle sandwich" because the man is "eating-out" the woman slowly (like a turtle would). The Japanese root of this phrase is there because Japanese people have a strong affiliation with turtles, especially in their diet.
Wegner (Teacher) - You're expose is late! You're getting an F this quarter!
Young Student - NO PLEASE! I'll do anything!!
Wegner (Teacher) - Even... a Japanese Turtle Sandwich?
Young Student - (gulps)... Yes! Even that!
Two vaginas are pressed togther with a thin layer of roast beef placed in bewteen! It is a thinly sliced lean roast beef pressed between two hairless clams.
Excuse me... is that a clam sandwich that you two lovely ladies are displaying? Yes, we are pushing our pussies togther and, yes, this is roast beef inbetween!
If you are one of those guys who likes to pleasure himself using comestibles of the meaty variety,then you would no doubt use a 'loose meat sandwich'...