Ok so it's not actually a toothbrush but it could be used as one at a push! The official line is :
Tooth Brush - Ergonomically shaped brush cleans cassettes and chain rings thoroughly. Shape allows quick transition from the front brush (angle cut bristles) or the end brush (long length bristles). Front brush is designed to be used on chain rings and outside of cassettes. End brush has long, larger diameter and stiffer bristles to pick the crud out of the spaces between cassette cogs.
