Some chainsets come with one-key release as standard. You can also buy them separately – they fit into standard crank threads.
These releases were invented so you could get the cranks off without using a crank extractor. They can also seize easily and can be fiddly. The one-key release comes in a bag with no instructions. The large diameter washer (usually black) fits under the bolt head; the smaller one (usually white) sits on top between the bolt and the one-key release cap. The white one is vital; it allows the top of the bolt to turn easily against the inside of the one-key cap. The one-key release cap fits over the top of this washer so that the washer ends up trapped between the bolt and the cap. Both washers should be greased to help the bolt turn easily when fitting and removing the cranks. Some one-key release bolts use a 6mm Allen key rather than the standard 8mm size. You’ll need to get an extra long one from your bike shop to get enough leverage, or you can extend a standard one by sliding it into a longer tube.
“Get maximum leverage, and tighten very firmly. Otherwise your crank will work loose and fall off”
Step 1: Start with the chainset side. When you come to fit the left-hand crank, make sure it lines up so that it points in exactly the opposite direction – this is easy on square tapers but takes a little more care with splined axles.
Step 2: Your one-key release kit comes with two washers. One fits neatly over the head of the bolt, the other sits underneath the bolt. Choose the washer that fits snugly over the threaded part of the bolt and sit it in the crank with a dab of grease.
Step 3: Thread the bolt into the axle, with the second washer pushed over the head of the bolt. This will also need a dab of grease to help the bolt turn against the one-key release cap when you come to take the crank off.
Step 4: Tighten the crank bolt firmly using an Allen key with at least 200mm (8 inches) of leverage. Hold the end of the Allen key and the end of the crank so that you get maximum leverage, and tighten very firmly. Otherwise your
crank will work loose and fall off.
Step 5: Grease the threads of the one-key release cap and the inside surface, and tighten into the crank threads. A small peg spanner is perfect for this, but you can also use circlip pliers. If you have the special Shimano tool for tightening the backs of chainset bolts (TL-FC20), it has a one-key release cap spanner on the other end.
Step 6: To remove the crank, simply undo the crank nut. It will turn easily briefly, but will then jam against the inside of the one-key release cap. As you continue to turn, the bolt will push the crank off the axle.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Removing and refitting the cranks : square taper and splined bottom brackets”