There's several ways of creating the fitting but the basic premise is that you are creating an under/through deck fitting by means of a fibreglass/xpoxt tube that loops from one hole to the next. Here's a link to the webite of G. Maroske. Not sure if this individual actually invented the fitting but his name is certainly attached to it:

http://www.studiofurniture.com/diary/kayak/outfit/moroske/

There are a lot of advantages to this type of fitting:

strong - you'd basically have to pull the deck line through your deck before it lets go.

streamlined - because the fitting is below the deck, they won't tear your legs off when you do a back deck scramble - not unlike some above deck fittings.

easy and cheap to make - a few dollars worth of epoxy and fibreglass and you're good to go.

low maintenance - no hardware to seize up or plastic to break

ease of refinishing the deck - because they are recessed, you can simply sand over them and then re-paint/varnish the deck without having to remove or paint around the fittings - unlike with above deck fittings.

Very neat appearance - your deck lines just disappear into the deck and re-appear magically a few inches a wy.

I don't think I'll ever go back to above-deck fittings.

Just my 5 cents worth,

Sean