One guy told me this is allowed. The server bends knees suddenly, gets a reaction from their opponent, causing their opponent to foul by moving their feet when they are not meant to move their feet. Is that allowed? I suppose it'd be breaking the rule that says the service has to be in one movement?
It depends on what exactly happens - all of service fault, receiver fault, or let are possible. First, let's review the applicable rules: Bending a knee can be part of normal technique too, so it's not faulted. However, the question of whether it could be faulted as a deliberate distraction per §13.4.5 is moot anyways, because §13.4.5 requires the shuttle to be in play. If the server seemed ready and bends the knee, it is very likely that they move the racket forwards a bit. If they stop the racket or pull it back before hitting the shuttle, that's a service fault. If no part of one of the receiver's feet stays stationary (e.g. the receiver makes a full step), then it's a receiver fault as well, i.e. a let. If the server did not seem ready to the umpire, or their racket did not go forwards, then the service has not started yet. If nothing else happens, the service has not started, and it's effectively a let. If the server serves immediately during the step, then it's a receiver fault. If the server only serves after the receiver has stepped, it's a let per §14.2.1, because the receiver is not ready. In practice, the likeliest outcome is a let. The receiver can easily just look at the racket movement, prepare to start once the racket goes forward, and only start once it hits the shuttle. In that case, the maneuver you describe is likely a service fault. If the receiver attempts to start overly early by looking at knees, eye movement, or whatever else, that's their problem, and they can be faulted for that.
No doubt though when people do underarm serves the racket takes a big swing back before hitting the shuttle. Do you mean if the server moves the racket forward(thus starting the serve), then moves the racket back, then moves the racket forwards, then it's a service fault? Presumably if they move the racket back and then forwards, then would say the serve started only when they moved the racket forwards? If we say a step has a start(receiver's foot comes off the ground), a middle, and an end(receiver's foot comes back to the ground). What do you mean when you say "after the receiver has stepped"? Do you mean after the end? Or do you mean between the start and the end? Would it be that if the server bends his knees and the receiver starts a step, and he serves, then is it a question of whether there was enough time for the server to be conscious of the receiver moving. If yes then both have faulted so it's a let. And if not then the receiver alone has faulted?
Correct, that's the very definition of §9.1.7, and also what I wrote (you stripped away the context in your quote). Yes, that is what §9.2 says, but only if both players are ready. The distinction is not made by the progress of the step, but temporally. For instance, just after the receiver has lifted their foot a little bit, they can still be considered ready to take the serve. On the other hand, if the receiver steps and then turns towards the umpire to complain, they are clearly not ready to receive. The rules do not give a clear definition of when exactly the transition happens between these two points in time. In principle, the receiver could be ready even after stepping, but usually, when they've landed, and the serve has not been struck, the receiver will lose tension and start talking, returning to their original position, or otherwise be not ready to receive. The only clear indication that the receiver is ready is if they attempt to return the serve (§9.4). Correct, this is an apt description, because when the server has realized the receiver has stepped, the umpire has presumably done so as well and considers the receiver not to be ready.
