Reach for your toes and crunch the abs hard.
Lift with your abs to reach higher, not by swinging your arms. Keep your legs vertical.
With your legs held vertical, crunching your shoulders off the floor and reaching for your toes makes the abs do a short, intense contraction against the fixed lever of your raised legs. Keeping the legs pointed at the ceiling means the hip flexors stay mostly out of it, unlike sit-ups, so the burn stays where you want it. It suits ab circuits and finishers, delivering a lot of upper-ab work from a small, controlled range.
No, and most people cannot. The rep is measured by your shoulder blades peeling off the floor with your abs contracting, however high your hands get. Reaching further comes from a harder crunch, not longer arms or a yanked neck.
You are pulling your chin to your chest and leading with your head. Keep a fist of space under your chin, eyes towards your toes, and lift from the ribcage. If it persists, do a few reps with your tongue on the roof of your mouth, it quiets the neck strain for many people.
Tight hamstrings are the usual culprit. A slight knee bend is completely fine and keeps the abs doing the same job. Straighten the legs over time as your hamstrings loosen.
