My coach was telling me when some players serve, they will do the usual prep and get the birdie infront of the racket head but instead of serving, they will move both their arms from left to right or right to left (keeping the birdie infront of the racket head) so you are not sure if the birdie will be hit to the T or out wide and then they will serve Is this legal??
Yes, that is legal. Donna Kellogg serves like this (in international tournaments), and I've never seen her called for it.