Add band tension, multiply the lower-ab burn.
Keep tension on the band, don't let your legs collapse together at the top.
The double leg raise already hammers the lower abs, and the band around your ankles adds a constant outward pull your legs have to resist while they travel, so the whole lower body stays rigid and the core never gets a passive moment. The slow lower to just above the floor is where the real work lives, abs fighting both gravity and band tension. It turns a bodyweight staple into loaded core work with one small loop.
Shorten the range, lower only as far as your back stays pressed into the floor. As your abs get stronger the safe range grows, forcing it early just trains an arch.
Yes, bent knees shorten the lever and make the movement much more manageable. Straighten the legs gradually as you get stronger for the full version.
Around both ankles or the arches of your feet, with your legs holding it taut at hip-width. If the band goes slack mid-rep your legs have drifted together and the extra load is gone.
