Michael Schacht has explained their usage definitely here at his Proa Rig Options: Crab Claw page, under the heading of Reefing: http://proafile.com/multihull-boats/article/proa-rig-options-crab-claw
(I really can't improve upon his description so I hope he doesn't mind me linking to it.)
They can be rigged leading to the mast to eliminate the need to go forward. I simply ran mine down to the heel of the yard and belayed them there to keep things simple.
My experience with spilling lines as used for reefing purposes is that they work well enough for short term use. They will reduce the sail area and get you out of a bind but the sail shape is not so great and your windward ability is seriously compromised. The sail also flogs a good bit - not good. But short term is OK.
For a longer sail in breezy conditions I recommend rolling up the big sail and flying a smaller sail, say 2/3 the size of the full sail. A smaller and flatter high wind sail works really well. One can easily carry the extra sail lashed across the outrigger booms and having a spare sail would be an especially good idea when cruising. On my 16' boat I carried a 90 square foot regular sail and a 60 square foot high wind sail.
All that having been said, I wouldn't be without spilling lines on my mainsail. They're another elegant and simple solution to a problem which was arrived at by the ancients. Here are some photos where spilling lines can be seen on the sail.
|Line can be seen on the lee side of this Carolines canoe|
|Lee side view|
|Overall sail shape is compromised but the sail is effectively depowered|