Edit: (Again) This DOES work for fall damage...
As stated below, you must have your camera looking up for it to work, so I will, once again, redo the instructions so that you will understand how to do prevent being dismounted this way.
This does not eliminate fall damage/damage, and does not always work. This also does not guarantee you survive a fall.
All this does, is allow you to keep the [Path of Frost] after taking damage, so you don't have to re-surface and re-mount to keep going.
If you've got a level 61 Death Knight, you know what I'm talking about. If you have [Path of Frost] activated, and if you take damage, your [Path of Frost]is removed and you sink underwater. You then have to take the time to re-apply it, jump above the water, and re-mount. This is a way that (Most of the time) will prevent that situation from ever happening.
You might be thinking, "Holy shit this kid is a lazy ****", but it's easy to do, and I truthfully hate having to take the time to re-mount, so here it goes.
I wasn't sure what section to put it in, so bear with me if you think it's in the wrong spot. You also may think this is "Common Sense" but I'm sure a ton of people haven't discovered it.
For this to work, you have to have an available "Frost Rune" to use during the process.
Instructions:
>>Anything in this colored text applies to if you plan to use this for fall damage.<<
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(For "Fall Damage"). Adjust your camera so that you are NOT looking down, I suggest looking up, but as close to level as possible.
1. When you realize that your going to take damage, put your mouse on the [Path of Frost] skill button. (Ss you go over the cliff, mouse over the button)
2. Right before (Less than a second) you take damage, jump with the space bar button (It might not be the space bar button if you've altered your key bindings). (Just after hitting the ground [At, or a bit before taking the fall damage], jump)
3. Take the damage (You will be in the air).
4. Right after recieving the damage (While your in the air), re-apply the [Path of Frost] skill.
5. Land your fall on your mount, and be on your way.
MAKE SURE!: (For Fall Damage) - Make sure you adjust your camera so that you aren't looking down, or you will fall through the water.
Your going to want to jump just before you get hit (Take fall damage), the closer the better, this will give you more time to re-apply the spell. You have to make sure that you use PoF just after you take the damage. Timing is everything here.
Good Luck to everyone, may you get to 80 soon.