It swivels in two axes so when the boat or riding mower, (Or God help us, Motorcycle) etc. rolls and pitches the drink does not spill.
I find that using a riding mower while guzzling Smithwick's prevents those annoying straight lines one gets by simply driving back and forth. It's more randomized and aesthetic.
Shrug. As good an excuse as any....
Then we shall be friends!
Cheers!