They perform the same function. They're solid inserts, threaded inside and out, rather than "springs" like helicoils. As fitted to later VW crankcases.
PS Also, they tend to be more robust than helicoils, which can come unscrewed (which leaves you a bit stuffed). Can use helicoils for case studs or things like snapped head studs, tho'. Stuff like stripped spark plug threads, inserts are better.
