In here I’ll also describe splits under the bar, also sometimes called low split.
Both of these are poses where you are upside down hanging on the bottom bar with your hands. The difference is in leg positioning: if they’re bent, you’re in a stag, and if you get them mostly straight it’s splits (to get a flat split you’ll need active flexibility). The body should be fairly straight. Alternatively you can try an arched variation, as long as you can control it.
An easy way to enter is directly by going from the floor to an invert and taking one leg through. Another option is from hocks, taking one leg back while holding on to the hoop.
Here is a demonstration of both moves, starting with splits and ending with stag under the bar: