In maritime vocabulary what is meant by full astern?

The term "full astern" is used by naval officers to instruct the engineroom onboard a ship to reverse the engines at highest possible speed. It's used mostly in emergency situations to avoid collision with another object, a ship or so. In modern days, the crew on the bridge do this by moving a lever, the engine room telegraph, to full astern and the reversing action of the propellor/engine is done electrically and automatic. If the telegraph or enginecontroller should fail, the engineers can control the engine manually, and receive directional orders by emergency telegraph or telephone. In the titanic steam engine era, it was done by setting the engineroom telegraph lever to full astern, but instead of the electrical system, a pointer on a similar telegraph in the engineroom moved to "full astern" and the engineer would reply by moving his lever to full astern, according to the pointer. In the wheelhouse, a pointer would move to full astern to show that the engineroom complies with the new order given by the bridge. After that, the engineer would reverse his engine by shutting the steamsupply to the engine off, reverse the valvegear on the engine and reopen the valves to let the steam flow to the engine. These old fashion telegraphs worked by steelcable running all the way from bridge to engineroom. One might think it is one way communication so to speak, but in both cases, the engineroom could move the telegraph to a slower speed, from full ahead to slow ahead etc., to alert the bridge, if an emergency occours in the engineroom, fire, medical rescue, mechanical failure etc.