Here is the explanation provided from Bosst Bottle Industries:

It's function is to absorb & release pressure pulses within the intake vein, assisting engine breathing. Now an explanation of exactly what goes on involves a lot of physics and a good understanding of fluid dynamics & pressure zones, but can be explained rather easily. During the intake stroke, air is sucked through the intake manifold and into the port at a considerable velocity. Because of this velocity, the air is said to have relative inertia. Now, when the intake valve closes, the kinetically charged air suddenly has nowhere to go, creating a high-pressure area. Naturally, the air wants to release its pressure in the easiest way possible, so it tries to shoot back out the way it came, creating a pressure wave in the opposite direction we want it. Then no sooner dose the wave begin to travel, the intake valve suddenly opens again leaving the motor to try and pull the pressure wave back in again. This dramatically effects low RPM, wastes energy and kills low-end performance. Now the same engine, but with a BOOST BOTTLE. Air goes whistling down the manifold until the intake valve closes. The air, still all charged up with energy tries to escape back up the manifold but is met with a small port that has a residual vacuum. This vacuum being caused by the previous suction of the intake stroke, sucks most of the pulse inside the Boost Bottle, storing much of the pulse’s energy. The intake valve then suddenly opens, creating a low-pressure area outside of the Boost Bottle. The gasses then shoot out of the Boost Bottle, directly into the intake system as a pressure pulse, instead of having to be pulled in by the piston. More air is then also pulled in from the manifold to completely fill the cylinder… Thus giving you an additional boost of power.

I was sceptical at first, but tried one and now have them on both of my 2-strokers. IMO, they are worth the money, but I do like getting everyone's experiences. The "boost" could just be in my imagination

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Any intelligent fool can make things bigger and more complex... It takes a touch of genius - and a lot of courage to move in the opposite direction.
- Albert Einstein