Welcome and I would highly disagree with essentially all of that. Yamaha or Rotax jet engines all work the same way, in that they only spin one direction, which is what allows them to be compact and need a reverse bucket to begin with. If they could change directions, you wouldn't need a reverse bucket. I highly doubt most of those people on that forum have some background in engineering and mechanical systems. I've been told this "trick" years ago by other jet boat owners, but they didn't know how it worked, nor did they have an understanding in mechanical systems or physics. Even with own dad, he insisted on this trick when he sucked in rope. Tired of arguing with him and told him to go ahead and try it anyways, at which point it didn't do anything and just continued sucking in more rope and tightening it up. Had to drift to shore and jump in the water to cut the rope off and slowly pull it out. And this was with a Yamaha Jet boat, not a SeaDoo. The drive shaft of a SeaDoo has a cover over the drive shaft, above the intake grate, that goes right up to the beginning of the impeller. With that in place, it makes a sucked in rope a lot easier to remove, as nothing is twisting it around.
What those directions sound like is not fixing anything. As I stated before, going backwards doesn't change the directions of the impeller. If it sucked in wood with the impellers going CW and jamming it, going in reverse would just continue forcing the wood against the impeller and wear ring. It's like screwing in a bolt and accidentally over torque and strip the thread and, to fix it, you continue screwing in the bolt the same direction and adding more torque, it just makes things worse. To fix it, it needs to be backed out, same as sucking in a piece of wood. The only way to do that is either reversing the direction of the impeller, which you can't do putting in reverse or with the engine on at all. Or, really the only option, is to get under and physically remove it.
When they say
"Put the throtles into forward (this allows water to be pushed directly into the exposed jet nozzles and force stuff in the tubes back through the grate, or off the grate)."
I have no idea what that is supposed to mean. A jet nozzle forces water out in one direction only. How would going forward, which means water is flowing out, somehow result in water flowing backwards and back out through the grate?? That's a contradiction on the physics of the engine. Even in reverse, the jet nozzle is still processing water and forcing it out the back of the boat. The reverse buckets happen to be dropped in the path of the jet stream of water. The reverse buckets are designed to redirect and bend that water underneath the boat, resulting in an overall force pointed backwards, allowing the boat to go back.
What that trick does is that when a piece of wood is wedged between the impeller near the beginning (closer on the intake grate side), your leading edge of the impeller blade will slice into it, pretty much like a knife, and wear out the end of it. Since your blades aren't really designed as knifes, it will dull and begin rubbing against it to the point it forces the wood to slowly rotate along the edge of the blade. moving it further down the path until it eventually it passed through the entire length of the impeller. And as it moves, it wears out all 4 blades and wears out your wear ring. That tight gap between the impeller and wear ring, needed for optimal efficiency and thrust, is now widened if the wood hasn't damaged the impeller blades. To fix the issue, you need a new wear ring and impeller. If the "trick" was never done, and just went under the boat, you could catch the piece of wood at the beginning of the pathway. May have slightly worn out part of the blade and wear ring, but really shouldn't be as impactful on the performance compared to the entire length of the blade being worn out along with the wear ring.
The best bet this "trick" does is try to dislodge whatever is stuck between the impeller and wear right so it drops down into the center, along the drive shaft, and force out through the gaps between the blades. But doing that may bend lead edges of the blade and potentially cause issues. Additionally, the likelihood of that happening depends on the size, shape, density of the foreign material. I say that because if you look at impellers, you have 2 different numbers. One number represents the angle of the blade coming off the center, while the other reflects the angle near the outer edge. The purpose of this is that, when water is processed, it creates a centripetal effect on the water, forcing it towards the outer edge and out. This is why items really get lodged between the impeller and wear ring. As its sucked in, it would be coming through the center of the stream, but once it encounters the impellers, and water is forced towards the edge, it gets carried along with the water, where it then spins and gets itself wedged. So even then, the "trick" wouldn't help dislodge it. If the foreign item is seaweed, just providing more throttle and increasing the RPM would be fine enough, just because seaweed is so soft and easy to cut through. Upping the RPM would allow the blade to cut through whatever is wedged in there and not thick enough to bend, dull, or damage the impeller blade or wear out the wear ring.
Hopefully this provides some more insight into how jet boats work and to not perform that "trick"
