Airsoft guns have ammunition magazines that resemble magazines of the real gun. But unlike their realistic counter parts, the internals of these airsoft magazines are available in various types. The trick for their use is different as well depending on the type.
For the uninitiated, most high capacity airsoft magazines has a winding wheel on the bottom (see photo below). Its purpose is to feed the airsoft BB's up the magazine into the airsoft gun. This magazine type is easy to load--pour the BB's into the relatively large hole--but not so easy to get it to feed right.
After loading the BB's, wind the winding wheel. One type of winding magazine allows you to wind it until you can't wind it any more (e.g. Echo 1 SG Task Force Black 1 MP5). The other type of winding magazine will continue to wind, but the winding click changes feel when it's fully wound (e.g. Cyma CM.023 MP5).
After winding the magazine, insert the magazine into the gun. A few BB's will load into the gun, causing the spring to unwind a bit. If you start shooting, you may get one or two shots, but then your airsoft gun will stop shooting BB's. Therefore, after inserting the magazine into the gun, be sure to wind it again.
Depending on the magazine, some magazines require you to continue winding as you discharge the BB's. Other magazines allows you to dispense the entire magazine on one winding.
Winding the magazine to load ammunition may not be all that realistic. But magazines without winding wheels are also available on the market. Generally, the low capacity magazines are all without winding wheels. There are also many mid capacity magazines that have no winding mechanism.
These no-winding magazines aren't as convenient as the winding wheel magazines, because there is no loading hole for the BB's. They requires the use of speed loaders to load BB's through the feeding opening. But once you load the BB's into these magazines, you don't have to wind it as you play.