The rules for normal (“safe”) dismounting are as follows :
Once during your move, you can […] dismount. Doing so costs an amount of movement equal to half your speed.
Some external factors might forcefully dismount you :
If an effect moves your mount against its will while you’re on it, you must succeed on a DC 10 Dexterity saving throw or fall off the mount, landing prone in a space within 5 feet of it. If you’re knocked prone while mounted, you must make the same saving throw. If your mount is knocked prone, you can use your reaction to dismount it as it falls and land on your feet. Otherwise, you are dismounted and fall prone in a space within 5 feet it.
But what if you don’t want to “safely” dismount (landing up), and instead, throw yourself off your mount, landing prone ? Can you even do such a thing – either via the above Dexterity Saving Throw (where failure is the desired outcome), or through an Acrobatics/Athletics check, etc. ? More so, would it cause damage to brutally throw yourself off a moving mount ?
The purpose of this (as I’m sure you will ask!) is for someone with the Athlete feat (or 6 levels in Way of the Drunken Master Monk) to spend less movement on dismounting, by throwing themselves off, landing prone, but then quickly getting back up.