No matter how you want to justify it, swinging erratically and kipping your toes up to the bar you're hanging from isn't an efficient way to build six-pack abs. Here's a modification to CrossFit's classic "toes-to-bar" that'll actually chisel your midsection.
The original exercise is done hanging from a bar. Most who do them have performance goals in mind and are wanting to smash out the reps. If that's your thing, fine.
But if you've got six-pack goals in mind, your focus should solely be on placing maximal tension through the abs. Since the rack toes-to-bar is done on the floor and in a more stable environment, it allows you to place max load through those tissues.
The best part about strict toes-to-bar and hanging leg raise variations is that they're brilliant lower-ab builders. While you can't isolate your lower abs, focusing on exercises that load a posterior tilt can help recruit more of these lower fibers. You can achieve some respectable external oblique activation from strict hanging positions, too.
That said, your abs are not hip flexors! So a large portion of the strict toes-to-bar (and hanging leg raise variations) – where you lift your legs from full-hang to around 90-degrees of hip flexion – aren't doing much for your abs.
It's that wink of your butt towards the top that you're mostly after. Sitting down allows you to work that portion of the movement the hardest, allowing you to hit your lower abs even harder.
The regular toes-to-bar can actually tear up your hands too. And bloody calluses aren't really a sign of a good workout. Ruin your hands and you won't be hanging from the bar for sometime afterwards. Rack toes-to-bar doesn't tax your grip so much, which is good because you're probably wanting to keep your hands fresh for whatever else you want to do with them.
Just treat you abs like you would any other body part. If you want to bring them up faster, then make them a priority within your workouts.
Aim for sets of 10-15 (quality) reps. When you're hitting 15 reps easily, then start adding some weight. It's one of very few exercises when those light ankle weights are actually a good purchase.
Here's what the loaded version looks like: