You can work your abs pretty hard with this one, especially if you maintain good form. The guy in this video has it pretty well nailed. The important thing is to avoid swinging - a wide grip helps with that, but it also takes a lot of additional effort from the abs to control the downward movement. If you just let your legs drop under gravity, you will swing wildly.
Engaging the lats is also important with this exercise, as you can help to stabilise your body and power your hips upwards by squeezing them and also use them to control the hips on the way down.
How straight you keep your legs affects how hard this is - I find I must keep my legs at least slightly bent to do even one rep - and to do 10, significantly bent. Simply bend your legs as much as you have to for the number of reps you want to execute. When you are very tired - as this guys is towards the end of his set, you can pretty much move to knee-raises. Or if you prefer, just raise your legs only as high as you can get them.