I only noticed this the second time, when it was definite and through repeated experimenting. I discovered it by accident by running down a cliff and not taking damage; and in addition, appearing to have not taken any damage.
I have only tested it with a Death Knight Bloodelf Female, with and without mount.
Step 1: Set walk on [Default /]
Step 2: Walk to the cliff and down it.
Step 3: ???
Step 4: Profit with no damage taken.
What it appears to be is a slow-fall action taking place.
This was done in the WoTLK Beta Build 8788.
It appears this only works for less than 80° cliffs.