I have two Ritchey Breakaways. One is a solo and one is a tandem. They routinely go up to 30 MPH on flat ground. The tandem has been known to go 45 MPH down hills. I believe that the answer to how they work is that the geometry of the couplers and the bike frames prevents flexing.

On the tandem front section of the frame is two sides of a triangle jointed to the middle section with two clamped joints. The clamped joint is conical on both halves. It can rotate, but it can't flex side to side. The clamp pulls the two cones together, and one of the conical sides fits inside the other. It could rotate, but only if the seat tube flexes, and it does not. The rear of the tandem uses one conical joint on the larger bottom tube and a keyed coupler where both halves clamp to the seat post as the pin which holds it together. The solo road bike uses one conical joint and one of the keyed joints which clamp to the seat post.



Also, I believe that the S&S patent dates from 1994, so you are free to copy it if you want. Ritchey's patent expires at the end of 2022.

The full explanation can be found at:
https://patents.google.com/patent/US...nt+no.+6886844