That works your abs, and is also surprisingly easy to have bad form (though it sounds more like this was just surprisingly exausting for you).
Tons of abdominal exercises are both surprisingly exausting and high injury potential (dragon flags @.@) but they look alpha as fuck so people still do them.
I personally lean toward the school of thought that favors less of the abdominals at once, and higher reps. I do actually like hanging knee raise as it's more the lower abs, but extending the feet (keeping legs straight as you raise them, I think this is what you were doing) makes it a much higher intensity exercise. I like planks, ball pikes, and crunches.
Many people feel differently about this than me. Part of that is because they have good form and like what they like / want to do low rep sets with everything (I do low rep sets with most things I do). I just no longer do any kind of pride lifting now. I don't do personal single rep maxes anymore. I love deadlifts / clean + jerk but I've stopped doing them.
TLDR: I think you thought something was a leg exercise that was abdominals, then I rambled about abs~