We'll begin at the beginning. While underway, the man in charge of the ship was the Officer of the Deck (OOD). Of course the CO is in overall command, but the details were handled by the OOD, especially when the CO was asleep. Courses, bell orders (speeds), and other such minutae was the OOD's responsibility. So, let's devise a scenario: the OOD wants, for whatever reason, to change the speed of the ship. This is no emergency, so there's no need to alert the CO; this is merely a routine matter. The OOD will give the new order to the lee-helmsman (as opposed to the helmsman, who is actually steering the ship), for example "All ahead two-thirds" or "All ahead flank". The lee-helmsman will acknowledge that order and operate the Engine Order Telegraph (EOT). Anyone who's ever watched the movie Titanic or any old war movie will be able to pick out the EOT in this photo. For the benefit of the cinematically-challenged, I have marked it in the photo of the pilothouse.
Time to dispel a common misconception: the lee-helmsman, when he operates the EOT, has absolutely no control over the engines. None. It's an Engine Order Telegraph; all it does is transmit the order to the enginerooms. The watchstanders in the enginerooms are the only people who can control the speed of the ship. Of course, our watchstanders in the engineering spaces are paying close attention to their EOT and notice when the order comes in; but just in case, there is a bell that rings when the order comes in, hence the term "answering bells" for getting underway; when the order comes in to the enginerooms via the EOT, the watchstanders have a basic panel copy of the EOT and they acknowledge the order and let the lee-helmsman know they are responding to his order.
Complicated enough? It sounds that way, but in reality it's quite simple. So now the enginerooms have a new bell to answer, let's look at how this is accomplished. There are four engineering spaces aboard CANNON-Class DEs. They are commonly called "enginerooms", which is technically incorrect. Two of them, the first and third, are enginerooms, they contain the engines, GM 16-cylinder diesels putting out 1700 horsepower apiece. There are two of these monsters in each of the two enginerooms. The shafts from these diesels actually go through the bulkhead to the second and forth engineering spaces, the two motor rooms. The four diesels turn four large generators, which are wired to four electric motors, two on each main shaft, and each main shaft turns one screw (propeller).
So, why was this ship designed this way? Several reasons. First, they couldn't hook the diesel up to the screw directly: a ship of this size needs a relatively lower screw RPM to be effective, otherwise it would just froth up the water and the ship would go nowhere. During peacetime they would use a large transmission called reduction gears to slow the shaft RPM to a usable speed. The problem is that reduction gears are difficult to produce (and therefore produced slowly). While the DEs were being built at breakneck speed (the record is 23-1/3 days), the builders couldn't wait around for reduction gears to be produced.
Secondly, reduction gears are expesive. Very expensive. So expensive, in fact, that to this day, the Navy does not buy the reduction gears we put in our ships: we lease them. When the ship is being scrapped, the reduction gears are inspected. If they're still good, they're put into a new ship being built.
Thirdly, this diesel-electric design means that if one diesel or generator fails, the other diesel-generator pair can turn that shaft, admittedly at a slower speed, while the other diesel-generator pair is powered off and repaired. So rather than running at a maximum of 1/2 speed while one of the two shafts is out of service, the ship can theoretically reach 3/4 of maximum speed, since one of the four diesel-generator pairs is out of service for repair. Additionally, there is no need to have all four diesels running at all times: while escorting convoys, one diesel-generator pair on each shaft would be more than enough to keep up with the brutally slow convoys, thereby extending fuel economy.
Speaking of fuel economy, that's fourth. Direct reduction gear coupling produced as fuel efficiency of about 70 gallons to the mile. No, not miles per gallon, gallons per mile. The diesel-electric setup cuts that nearly in half, to a mere 35 gallons per mile. In wartime, when resources are stretched to the breaking point, this is a crucial point.
Also in each engineroom is an additional 8-cylinder diesel. These two diesels are connected to another, you guessed it, generator. These two generators supplied all the electrical loading for the ship, lighting, winches, electric ranges, everything. And, just to be safe, there is a 3-cylinder diesel that is an emergency electrical generator, if everything else fails. I can't confirm this, but I have a feeling that, in a pinch, there was a way to cross-connect the generators used for propulsion into the power system, but I'll have to do a little more looking into this theory.