The shield was an added item for use in the aMazer editor. It was announced that it would be added in to the new aMazer version, and according to the news section, it was released for use on August 8, 2009.
It is usually used in multiplayer, along with a slingshot and crumbs. You must have a shield and a loaded slingshot to do or take damage against a player. Once a shield is depleted, a player is automatically placed at the beginning of the level. Any items they had, such as a jetpack or slingshot, are gone. Shields come in quanities of 5, 10, or 20 hits. They may be glitchy, as some players pick them up and the game crashes for them, or the amount of hits a shield gives can double when picked up.
In single player, a shield will stop Icemen from freezing you when they touch you. Furthermore, chasers will not slow you down if you are equipped with a shield. However, once your shield is depleted, the effects of an iceman and a chaser will return.
They are the simple building block for a PVP game (along with the slingshot and the crumbs) to have multiplayer battles with 2-20 players; all players have to hit each other a specified number of times, usually in an arena with icemen which can make a hard but fun game. It is often more fun and realistic when 5-hit shields are used.