"Butt scratchers" are named after how a dog looks when trying to wipe its butt on your carpet. If yours does this frequently, you may want to consult a vet. For humans though, mimicking this butt drag (except going backwards) is a surefire way to pummel your abs.
Here's what they look like:
The idea is to tuck your pelvis under, behind your arms. Your butt isn't touching the floor though; it should be as far off the floor as your arm length will allow.
This "drag" movement makes these particularly useful at working your abdominals through both spinal flexion and posterior pelvic tilt. This combo means that butt scratchers are effective at working your superficial abs in their fully-shortened position.
There's zero abdominal load through most of the movement, but as soon as your butt drags behind you you'll be lucky if your abs don't cramp from the intense contraction. Try to hold it there for a brief second before letting your abs up for a breather.
Awareness of your superficial abs can often be an issue when trying to develop them. You need to actually find them before you can target them with more loaded ab exercises that'll make them grow. Butt scratchers will help you find your abs with very little load and have them wrecked after just a few sets.
If you're the type that feels abdominal crunches in your neck, or leg raises in your hip flexors or lower back, then butt scratchers are a novel alternative that'll get the job done.
No Sliders or Space? No Problem!
If you don't have a pair of sliders, don't worry. For a fraction of the cost, a pair of furniture sliders will do the same thing and will slide on most surfaces. You'll find these in many hardware stores or any gigantic online retailer named after a rainforest. An old towel will also slide comfortably on a wooden lifting platform or gym floor.
As an alternative, butt scratchers can also be performed statically. Your hands stay in one position on the floor and you'll rock back and forth.
Again, the focus should be on tucking your pelvis under and behind your forearms. Statically, these can be done with your heels in a suspension trainer (TRX, etc.), on a foam roller, or with a Sorinex glute-ham roller or similar.
If T-Rex arms let you down, an easy workaround would be to have something by your sides to elevate you higher. Push-up handles, yoga blocks, a few hex dumbbells, or step boxes will work.