For descending, on some stairs I can descend while "falling", it's very fast and not tyring it all.

You have to land on the edge of the stair below so you "drop", then rinse and repeat.

If you do it constantly, you only have to support your body ( but you use your weight to fall, so don't support it too much) and slide with your hands.

If you can't do it the first time, land with your foot normally and then always turn them slightly to go to the stairs below

Kinda hard to explain