Ah, this is a fun hyung, and a fun move.
My advice is to do exactly the opposite of Tez3's advice, at first. Practice it slowly, in this order:
(1) From the two-fisted block/uppercut in hugul jase, snap both fists to your right hip, right on top, while you simultaneously bring your left knee straight up to your chest. Practice this part especially, making sure you get the snap before you go any further. Also practice standing in this position for more than a second or two, just to work on balance. Keep your right knee slightly bent -- in other words, don't stand up or raise your body at all from your hugul jase.
(2) Once you have that part down, continue to the next part: slam your left foot back down into the ground, propelling yourself upward as you bring up your right knee, all the while keeping your hands at your hip in the same position. You shouldn't be going forward more than a front-stance length or two; too much forward momentum will throw off your balance, and you want to use gravity here anyway. When you land, land with just your right foot. Your right foot should be pointed slightly to your relative left, at about a 45-degree angle off the direction of your previous motion. Again, don't have your right knee locked out; keep it slightly bent, for balance. Stop here, just after you've landed on your right foot, and practice up to there a few more times.
(3) After you have all that down comfortably, add the last part. Your hands are still at your hips, and you're balancing on your right leg. From here, just drop your left leg down behind your right one so that your left shin touches your right calf and your left foot is directly behind your right one, pointing straight towards what was your left when you started. Get that position first, both your knees still slightly bent.
(4) Now, pivoting back your right hip slightly, snap your hands down into a low x-block in front of your right knee, dropping your butt straight down, your back straight, your knees bending low. You're sitting down, basically. Your right arm should be on top of your left. Also, your right foot may pivot so that it's facing in the same direction as the left, so they're in alignment. You can try landing it that way, but it might be a bit more awkward, and you'll lose some snap. Practice (3) and (4) a few times, from set down to snap, until it's all comfortable.
(5) Now go back to the uppercut in hugul jase and do (1) through (4), this time making the left foot and the block land simultaneously( <-- important ). Remember to keep your back straight, getting low by bending your knees and dropping your butt as opposed to bending forward. It should be pretty comfortable, for a few moments, at least. Keep practicing it until it's all fluid.
I'll post a video of this as soon as I can, if it'd help anyone, and if none of my seniors in TSD don't have one readily available.
Tang Soo!