On the Waist Max machine, pulling the knees up and in toward the chest while curling the torso forward compresses the abs from both ends of the movement at once. That double-ended pull is what separates a tuck crunch from a standard machine crunch that only bends at the torso.
Log your tuck crunch waist max sets in 1RM.fit and watch your estimated 1RM update automatically as you get stronger.