The ab rollout is an anterior core movement done with an accessory called an ab wheel, or ab roller. Starting either kneeling or standing with their hands on the handles of the ab wheel on the floor in front of them, one should brace their abdominals in order to create a slight spinal flexion and maintain that position throughout the movement. Their arms should be maintained in an overhead position, so the upper arms should be by one's ears, and the scapula should be depressed. From there, one would push the wheel as far out in front of them as they can while maintaining braced abs and a slight spinal flexion. Ideally, the movement would go far enough for the hips to go from a retroversed position to a neutral position, then one would come back up by pulling the wheel back towards one's feet using a strong hip flexor and abdominal contraction.
Here are a few video examples.
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I have not looked for good tutorials for this movement yet, but first of all you should work on getting the prerequisites I mention below. Next, you can use either a proper ab wheel, or a dumbbell, or anything that can roll on the floor and support your hands, really. Even low hanging rings, or swings, can be used for that kind of movement (the physics of it changes a little bit if using rings or a swing, but I'm not going to talk about it much unless someone asks). You should first try to do them kneeling down. Use padding for your knees if you wish. Placing your hands onto something higher then your knees or feet will make the movement easier. Placing them on something lower, or elevating your knees or feet, will make it harder. If you have the prerequisites and struggle to do it kneeling, you can use partial reps (stick to 5 reps, but focus on always trying to go a little deeper) or change the height difference between your feet/knees and hands to adjust the difficulty. Once you can do it well, work on increasing the amount of reps you can do, or intensify the movement by changing the height difference, adding weight or using bands to resist your movement. You could also use your arms to make it a little easier on your core, like in the first example video. You can use the same principles to transition to standing ab rollouts. Partial reps, height differences, using the arms, etc.
-Being very comfortable with crunches (you should be able to do 20-30 without much fatigue)
-Being able to hold a clean hollow position (I suggest at least 30-60 seconds)
-Being able to do a few pull-ups (we use our lats to help bracing the shoulders when we do ab rollouts)
Feel free to use this topic to discuss the movement some more.