The new foils increase performance by reducing the virtual displacement of the boat and improving vessel motions.

The new foils generate vertical lift that offsets the weight of the vessel with the trade-off of producing less side force opposing the sails. The idea being that the lift reduces the amount of weight supported by buoyancy, which creates a reduction in "virtual displacement"- basically the water thinks the boat is lighter.

Displacement has a big impact on overall resistance of a boat, so with the new foils there is a potential to drastically decrease the drag produced by the hull.

Simplistically, you can break it down into drag caused by pressure (think pushing the water out of the way) and drag caused by friction (the act of water sliding across the hull). The boat being partially lifted out of the water by these foils reduced the amount of wetted surface and frictional resistance scales to wetted area, along with length and speed.

Pressure drag further breaks down into form (i.e. parasitic or induced) drag and wave making. Form drag scales roughly with frontal area coefficient (the area of water the boat needs to move through) and lifting the boat reduced this. Since boats operate at a free surface (interface btw air and water) energy is further dissipated from the boat to create waves. Wave making resistance scales roughly with displacement to length ratio. Again, the foils reduce that.

Finally, there was talk about the new foils working to lift the bows of the boats out of the water to improve vessel motions. It appears the foils are doing that, keeping the bow free when the skippers are pushing hard which keeps them from digging in and sending green water over the deck (both slow a boat down). However, from what I've heard from skippers the foils make the boats behave more violently which is not as nice. So, maybe the jury is still out on that one...