Usefull to get inside walls/buidlings/rocks, pretty much anything
Use a mount and run up an incline on whatever you want to get inside, when the mount looks like it's pointed to the sky, stop running. turn in place and if you have a good location it'll spin so that your body goes inside of the object. After that its a bit more tricky and needs some practice, you dismount (and Ive found that crouching helps) you fall inside of the object. after that simply de-summon your mount and bam! done.
Don't use this on hills or any type of terrain that isn't an object, since you'll just fall through the world