This is a pair of replacement knees for PotP Grimlock that add more length to the thighs for better proportioned legs. This also has the benefit of adding range to his tail bend, which lets the dinosaur mode stand up straighter.
Installation involves the complete disassembly of the legs. Keep track of which screw comes from which hole because they can be slightly different sizes(the thigh screws in particular vary between the top and bottom hole). Afterwards you have to pop the tiny glue points loose on the lower legs by carefully prying them apart( IIRC they're in the front of the knee, the shin, and the back of the calf), which may snap a small connection peg or two off. This won't matter if you use a little bit of glue on these pegs when you reassemble the legs, which I recommend for a better closure.
Make sure the black combiner port clip is properly inserted and held in by your finger during reassembly. It has a little lip that needs to slip through the rectangular hole between the peg slots and grab the outer side slightly. This is extremely important if you want the combiner mode to still work.