I and my friends got into a long discussion on this topic. Need someone to clarify here. I saw in some tournament some professionals serving motion; both arms are sliding/moving/aiming horizontally, then serving. And the judge never complained their serves. What I did is from right to left, then left to right, then serve. Is my serve legal? The rule says the serve is starting from the forward motion only.
A lot of small details goes into if a serve is legal or not, so it's hard to say if your serve is legal without actually seeing it. But what can be said is that it is possible to make a legal serve where you move the racket and shuttle horizontally before serving, as there are no rules against this movement.
Let's review the laws: In other words, as soon as you are ready, the first forwards movement of the racket is the start of the service. After that, you must not move your rackets backwards. If you always swing around before serving to find your perfect spot, and then look at the opponent (or otherwise wait before serving), then it's no problem, you are simply not ready until you reach your final position. But my understanding is that you probably only start swinging around once your opponent is ready to receive, and you serve immediately while swinging. This is far more problematic, since it could be argued that you are already ready when you start swinging. Deciding whether a fault has been commited would then mean judging whether you move backwards after moving forwards, which is very likely the case. However, the general consensus among umpires is that this law should be interpreted liberally. There are athletes who are regularly playing with service judges, but do not get faulted even for double-action strokes. The key of these is that the action is consistent for the player, so they don't gain an advantage (although this has no effect on the laws, of course), and are thus judged ready only once the second part of the action begins. Similarly, swinging around is usually interpreted as fine, even if you just stop for a second before serving. If you, however, manage to serve while swinging without any discernible action, you would most certainly be judged ready once the swinging starts, and thus be faulted if you swing back and forth. Interestingly, the German laws (which are supposed to be a translation) are a little bit more strict, inserting a comment noting that it's not only forbidden to stop, but also to slow down the action. In practice though, in my career as an umpire (about 20 tournaments so far), I only faulted once for double action, and neither I nor any other umpire or service judge I saw ever faulted for swinging. In summary, your service is likely legal, with the caveat that there should be a short stop before you serve - if you don't stop, but serve from the motion, it may be a fault. As always, if you disagree with your opponent, request an umpire from the tournament managers. I've been asked to umpire twice during tournaments where I played, and I can only recommend it to players who face any disagreements.
It is legal as long as you are not moving your racket forward towards the shuttle (would be considered as start of your service). I think Mathias Boe is well known for moving his racket/shuttle (in sync) left and right before actually serving.