HOUSTON — At 53 feet tall and 325 tons, the blowout preventer that occupies center stage in the worst American oil spill in two decades is a huge stack of equipment. But it is also a jumble of contradictions.

It’s simple: a brute monster capable of strangling a pipe, or in more desperate circumstances beheading it, to block the flow of oil and gas at enormous pressures from a formation deep underground. But it is also complex, controlled by an elegant brain of elaborate circuits custom built from pipes, not wires, and using hydraulic fluid instead of electrons.

Most of the time the device, commonly called a B.O.P., does little but sit on the seabed as drilling pipe and liquids travel through its main bore, which is usually about a foot and a half wide. But in the occasional frenetic moments when the B.O.P. is called into action it must work. Ordinarily no one ever hears about blowout preventers. Now the world has.

Blowout preventers are essential for drilling on land or underwater, and the rig accident has prompted talk of improvements, like use of modular parts or better materials. And the broader application of different drilling techniques may help operators become less reliant on the devices.