Minecraft wiki entry for Trapped_Chest, associated text: powers any adjacent redstone dust, including beneath the trapped chest, to a power level equal to the number of players accessing the trapped chest (maximum 15)
powers any adjacent redstone repeaters facing away from the trapped chest to power level 15
strongly powers any full solid opaque block (stone, dirt, block of gold, etc.) beneath the trapped chest to a power level equal to the number of players accessing the trapped chest (maximum 15)
activates any adjacent mechanism components, including above or below, such as pistons, redstone lamps, etc. Due to hoppers being locked by redstone activation, hoppers below a trapped chest do not take items from it while it is open.