Rope Clutches
In the hierarchy of fixing ropes under tension, we have cleats, then jam cleats, then cam cleats, then line-stoppers, and ultimately rope clutches.
A cleat is simply a two-pronged thing, fixed firmly tothe body of the boat, around which a rope or line is wound and then made fast. It is difficult to make fast a line that is under tension, and fine adjustments are also difficult. Very strong however.
A jam cleat is a narrow, usually v-shaped channel, into which a rope is jammed against forward-slanting teeth or ridges - the tension of the line keeps it jammed. Fairleads my be required to keep the orientation of the tension of the line in the same plane as the jam cleat. Not as strong as a conventional cleat, they can be released more quickly, and are relatively easy to adjust under tension.
Cam cleats are capable of very high loads, and are just as easy to release under tension, and a bit easier to adjust under a load.
Rope stoppers, rarely sold these days, are an enclosed cam cleat (or just one cam facing a compression block). These are very strong, but difficult to release under tension. Operated by a lever that lifts the compression block.
Rope clutches are also enclosed devices, but are designed to be very easy to release under tension, while capable of high loads and suitable for making minor adjustments.
Sailnet has a lengthy article on the subject.
Antal Marine Equipment. Deck fittings, Rope Clutches and blocks.
Barton Marine Deck Hardware and General Chandlery.