The best I could come up with was a NY Times review by Caroline Weber (one of my go-to authorities on late-18th century politics and sex) of a book by Kate Williams called Ambition and Desire: The Dangerous Life of Josephine Bonaparte. Weber says it's supposedly akin to Seinfeld's "swirl", but I'm not a Seinfeld fan so I can't even begin to describe what that would be.

In general, Napoleon does go to some lengths early in their relationship to extol Josephine's sexual powers over him, so this could be just his way of describing what she was doing, not so much a "technique".

I've come across this as well. In Napoleon: A Life by Andrew Roberts there is mention of these zigzags but doesn't shed any light. Same story with Patrice Gueniffey's biography.

What is known is that Napoleon was fairly inexperienced compared to Joséphine when they began their relationship. He lost his virginity to a prostitute and is portrayed as rather clumsy in the bedroom in this period of his life. He was “baffled and excited by her repertoire of techniques” according to Kate Williams in Ambition and Desire: The Dangerous Life of Josephine Bonaparte. Now, consider the zig-zag motion itself: going in one direction and then pulling towards another. Perhaps a fanciful term for vigorous intercourse used by the ravished Napoleon? I do not think historians have developed a definitive answer (considering the intimacy of the sources I doubt much explication was needed between the lovers on this term) but I would love to be proven wrong. I would also love to know why Napoleon referred to her private parts by the name "Baron de Kepen" but these little secrets between people are why history can be both such a fascinating and yet frustrating endeavor. Disclaimer: I am not a professional historian.