In my listing, it's listed as "(any Courtesy Turn) By".

Examples:

Right & Left By

Turn & Left (Thru) By

Grand Chain Eight By

"By" means Cast 1/4 More (to a 2FL) and Slither.

"By By" means Cast 1/4 More, Slither, and Slide.

"By By By" means Cast 1/4 More, Slither, Slide, and Slither.