I bought this so that I could use an 11-36 tooth cassette with a Shimano 105 GS rear derailleur on my gravel bike. It was very easy to fit, only requiring a 5mm allen key.
Fit this between your frame and rear derailleur to accommodate a larger rear cassette. I fitted a 36-11 cassette and switched 105 short cage mech to a 105 GS medium cage mech. Added 2 links to chain, re index gears. Works perfectly with my specialised crux 46-36 front end. It’s no substitute for fitness and loosing weight, but it does help you to keep spinning on the Lake District passes!