Green’s Dictionary of Slang

(US) the navel, in the context of oral sex.

1966		Trimble 5000 Adult Sex Words and Phrases 175: rest stop (Vulg.) The Navel, in an obvious reference to progressive Oral-Genital relations which begin at the mouth or neck or Breasts.
1972		B. Rodgers Queens’ Vernacular 171: rest stop the navel as offering focus to an oral copulator.