Speed Based Fall Damage changes fall damage to be dependent on the speed that you hit the ground rather than the distance you've moved while airborne.
It has been carefully created such that falling off of a tower X blocks tall will cause the same amount of fall damage you would get without this mod.
It is possible to calculate the speed at which a player will hit the ground when dropped from a certain height. This mod reverses that formula, instead calculating the height a player would have to have been dropped from, based on their speed when they hit the ground. This calculated height is converted into damage through the normal fall damage formulas, i.e. damage = blocks fallen - 3
This mod is not suitable for competitive pvp because it puts clients in full control of the fall damage they receive. Due to how wildly player motion can differ on clients and servers, this was the only way to make the mod usable.
Other mods and vanilla features that work directly with the impact upon falling, such as landing on hay bales work without issue.
However, other mods and vanilla features that reduce fall damage by altering players' accrued fall distance, such as teleporting with an ender-pearl while falling will no longer work.
You can see the mod at work in this gif: https://gfycat.com/LeftCompassionateGossamerwingedbutterfly
Blood Magic's Air Sigil launches the player in the direction they're looking. In the gif, you can see how the sigil can be used to reduce fall damage by decreasing your downwards speed. You can also see how using the Air Sigil to launch yourself directly into the ground is now fatal, since you hit the ground at the same speed as if you'd fallen off of a tower about 25 blocks tall (25 - 3 = 22 half-hearts of damage).