I first heard of this effect when working on a Lürssen T45 FAC, which had basically the same hull as the S Boot. But the principle is explained in the link you posted😉-
"These rudders force the water under the hull outward, lifting the stern, thus reducing drag, and lowering the wake height, which “requires less energy, allowing the vessel to go faster."
"The effect was noticed at speeds above 25 knots, by swinging the "effect" rudders outboard by 30°. Once attained, the effect rudders could be swung back to 17° outboard, and the speed could be reduced to as little as 20kn while still maintaining the benefits of the effect."
"During test runs with S-2, the first boat from series production, it was accidentally discovered that the boat stopped responding to the rudder at high speed and full rudder positions but started to display the effects described above. When investigating what caused this behaviour, the basic principles of the Lürssen effect were discovered. Starting with S-2, the following boats were then equipped with the two small rudders on either side of the main rudder which could be angled outboard up to 30°.
A wedge was added under the rear of the hull from S-18. The wedge further added to the effect of helping the craft to stay level at high speed."
Alles klar Herr Kapitän?
Cheers, Doug 😎