A figure eight knot is a stopper: it puts a bulky lump in the end of a rope so the line cannot run out through a block, a fairlead or a clutch. Over, under, through — three moves. It holds more bulk than an overhand knot and, unlike an overhand, it still unties after it has been slammed tight under load.

“Over, under, through.”



Before you leave the dock, walk the boat and check that every sheet has a figure eight in the end. It costs ten seconds and saves the afternoon: without one, a flogging jib sheet whips out through the fairlead and you are on the foredeck in a blow re-reeving it.
Same for halyards. A halyard that runs off the winch and up the mast means somebody is going aloft. Charter briefings skip this; good crew do not.
Outside sailing you will meet the same knot in climbing, where the figure eight follow-through ties the rope into a harness.
As a stopper in the end of a rope so it cannot run out through a block, clutch or fairlead. In climbing, the follow-through version is used to tie the rope to a harness.
Yes, for this job. It has more bulk, so it jams less easily into a block, and it can still be untied after being loaded hard — an overhand knot under load can be impossible to undo.
Bend the rope sharply either side of the knot to break its shape, then push the loops apart with your thumbs. It loosens far more easily than an overhand knot.
No. The plain figure eight is a stopper tied in the end of the line. A figure eight loop (or follow-through) makes a fixed loop and is the standard tie-in knot in climbing.
Reading a knot and tying it on a pitching foredeck are different skills. Test the first one in two minutes, then close the gap with the course.