He's probably scent marking, but it's so darned cute.
This is Frank saying Wiggle Squiggle in sign language. There's no official sign for the word, but the visual says it all. Frank's fingers are wiggling like Caesar's legs.
Either that or Frank's signing something about a bug that has died and is kicking his legs one las time before he bites the dust. Happy Dog vs. Dying Bug... It's one or the other; both signs look the same.