These are not necessarily the best choice for ab exercises if you consider how “safe” they are (I don’t give any of them to my personal training clients). However, I do them once a while when I want to feel “burn” after the training.
It is almost like doing a crunch with bringing the legs up simultaneously. Make sure that while bringing the leg down, your hips don’t move along with the legs. That creates excessive lumbar lordosis and it is not good for low back health.
As the name says, you lift up your legs alternatively as your upper body does crunch. If you see this ab exercise on Youtube, some gymnasts perform very quickly. They need this quick movement for generating explosive power during their routine but if you just want to work on your abs, do it slowly. Ballistic movement is not good for your body at all. It is safer for your low back health.
I think this is the most controversial one. Lifting up both of your legs up to start, then slowly going down from there to work on your abs. First of all, if you are a big guy, don’t do this at all. Look how much weight your cervical spine has to bear. It is not good for your neck at all. Secondly, if you have a tight neck, you may feel more uncomfortable after this exercise.