To put it simply most of the benefit comes from simply lowering the rig. The mast head typically comes down 200mm for the “normal“ rig height while maintaining max luff length. A small amount of area is borrowed from the back of the sail to make the part that “sweeps” the deck. To actually get this to happen you need a deck fairing but not having one has little or no effect on getting the benefits.

To make a decksweeper work you need the distance from the top of the foredeck to the mast tip to be around 5200mm. Ideally the boom will be fitted to a mast gooseneck fitting with the bolt countersunk or very close to flush.

The opening for the hounds and spreader are usually placed in the same position relative to the top of the foredeck so, for example, if the mast is 200mm shorter the mast fitting are 200 closer to the mast tip. 

To work out what mast length you need simply measure the height of your king post and deduct it from 5200m

When to go sweeping or conventional?
So far it seems as soon as you are solidly on the foils the decksweeper sails are faster. But in marginal foiling conditions conventional sails are earlier up. The combination of the mast tip been higher (in less ground effected air),

and having more to lean against in the black art of coaxing the boat in to the air, does offer earlier foiling opportunities.

There are also questions around how aero efficient the bit of cloth under the boom is compared to having it on the back of sail.