Not too many players use rotation cancels and they are poorly understood by most of us. They are used to fire multiple LT shots in rapid succession, or to stop walking and fire any turbo shot as soon as possible. Stationary non-turbo shots do not need rotation cancels because they can be fired as soon as you let go of a walking direction.
After walking in a direction for even the tiniest bit, the length of time you must stop and wait before you can fire a turbo shot is equal to the length of your VR's deceleration animation in that direction. Firing multiple LT shots in succession by walking briefly in between is known as walk canceling.
After walking, you can instead input stationary rotation (not walking+rotation together) to stop yourself. Depending on the direction of your walk, inputting rotate does not cause you to decelerate any faster. But once you reach zero velocity, you'll be able to fire a turbo shot without waiting for your deceleration animation to finish.
The length of your deceleration animation should equal the time it takes to reach zero velocity, but the latter is quicker if your VR was changed from a previous game version, or you weren't moving at full walking speed (for example in between firing multiple LT shots). From a standstill most VRs can tap walk for up to several frames and still stop instantly.
After rotation canceling, if the followup is a standing turbo shot, there is an additional 7-frame delay that gets added into the shooting animation. If the followup is a crouching turbo shot there is no additional delay, however you must go from stationary rotation to crouch without hitting diagonals along the way. On twinsticks you may have to pass through neutral.
What is quickest to chain LT shots together, jump cancels, walk cancels, or rotation cancels? Jump cancels (including input frames) are 9 frames, 11 frames, or 13 frames for most VR. The shortest walk decelerations in the game are 8 frames, so the quickest walk cancels take 9 frames. Because of the 7-frame delay, using rotation cancels yields an 8 frame gap.
Rotation cancels are thus the fastest, but only by 1 frame on some VR. Fortunately there is some leeway in the timing of each step. You can start rotating the instant you let go of a direction, but it only matters that you rotate when you stop. If you rotate early, depending on VR it'll be okay even if you input walk+rotate together for a couple frames.
You can also can press the trigger anytime within the 7-frame delay, and a standing turbo shot will not come out any slower. In fact it's better to press it later because you'll get free rotation. This step is way easier to time than a jump cancel or walk cancel, where you have no such leniency.
Chaining crouching shots depends on the recovery of the particular attack, as crouching LT attacks can cancel into a jump, dash, or crouch slide earlier than they can cancel into a walk. Either jump or rotation cancels could be faster. You don't have to walk first, you can just rotate then fire again, but it only works if you wait until you are free to walk.