This part doubles as a crank extractor, so you can remove the cranks with just an 8mm Allen key. The crank cover is aluminium, rather than the standard plastic, and it is screwed into the cranks. The edge of the crank bolt head sits under the crank cover, so that as you undo the crank bolt it bears on the inside of the crank cover and pushes the axle off the crank.
With this, it’s even more important to have a good, long Allen key; you won’t be able to undo the bolt with an ordinary-length key. Use the Allen key to undo the crank bolt. It will be stiff at first, then will turn quite easily for a short way, and will become stiff again as the crank bolt begins to force the crank off. Keep turning, and the crank will be pushed off the axle. You should take the one-key release gadget off periodically and grease the parts. These tools will only work if the threads are well-lubricated and the parts are clean. There are two small holes in the crank cover that you turn to remove the cover. You can buy a pin spanner to do this job, although a pair of needle-nose circlip pliers also work fine. For fitting and removing details.
“These tools will only work if the threads are welllubricated and the parts are clean”
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Removing and refitting chainsets”