(“Ultimately you want . . . “)
Sneaking up on your horse (figuratively) and shouting “Boo!” (figuratively) is not generally the best way to prepare for a movement. Often whatever you’re trying to do fails at the outset due to lack of preparation. I can think of many cases where because the aids are “slapped on,” your horse can’t figure out what you’re asking him to do.
As a consequence in training situations we often devise fairly elaborate exercises to “warn” him, both physically and mentally about what we are up to. A horse which has trouble taking the correct lead, for instance, can be placed in shoulder-in or, sacrificing straightness momentarily, in travers to position him correctly and to place your weight and your legs where they need to go for the depart. To make it even more obvious for a confused horse, sometimes you can get the lead by spiraling in to 10 meters from a 20 meters circle and then leg yielding out to the circumference, making the strike off into the outside rein just as you get there.
Please understand, though, that these are training exercises, and ultimately you want a smooth, prompt, straight, and clean depart without the need of much fiddling around beforehand.
What must begin as overt and blatant should always be refined to become practically invisible. This goes back to my vending machine analogy. By proposing tiny intimations of each preparation that are only meaningful to you and your horse, you make sure he is ready for whatever you want. Put those large preparations on the shelf when they are no longer necessary.