Drill extensions enable your standard bits to reach out and bore something that they couldn't otherwise reach.
FAMAG 1639 extensions are designed to work with Bormax forstner bits but can be used with any drill that has an 8 or 10mm shank. Both types have a 10mm shank at the top so you can connect extra 10mm extensions if necessary.
FAMAG 3511 drill extensions have a standard 6mm hex chuck, so they can be used with any standard hex shank drill bits, screwdriver bits etc. The outside diameter of the chuck is 16mm.