Pressure blocks are an effective way to set up traps, they act like pressure plates however they're applied to the block itself rather than placed ON the block, this makes it almost impossible to detect and it can't be washed away by water.

It's simply made by placing the block you want for it to look like and a pressure plate of any type in a crafting table.

Like a pressure plate it clicks down when stepped on, giving the block a height pretty much the same as soul-sand.