Something doesn't look right but it's hard to figure out what it is. The factory picture does have a bolt or pin in the hole behind the pivot point, but imagining the pressure on the shank in use would seem to make that pin useless except for lifting it out of the ground. I hope someone with experience with these will chime in.
Another thought, it seems you've got that top link out awfully long. You may have to put that subsoiler in the ground to know if it needs adjusted.
Adjust the top link as short as possible; the "missing" bolt is a shear bolt.
When you lift it after shortening the top link, the subsoiler should "fold" and raise the foot -- it will be under the drawbar. The foot will be parallel with bottom of the tractor with about 6 to 9 inches of clearance. Maybe less with the smaller 790.
This implement is for breaking the hard pan or clay layer about 24" or more below the soil's surface. It improves drainage, and water reserve. On the larger field plows it usually runs through the rows where the actual planting will happen; this helps with water reserve and deep roots for the row crop; I know a farmer who does this every time he plants, his yields are greater than some of the other farmers in the area who do not; google / research: soil compaction, no-till, low-till, subsoilers, parabolic subsoilers -- read more about it. I know very little about anything.
Figure 40hp per shank; the larger subsoiler units with 5 or more shanks need 150+hp tractors to pull them; from what I read, the parabolic shaped units can be pulled with less HP. See here as an example: http://www.monroetufline.com/products/subsoilers-series.htm
One last comment, use it during the dry part of the year / season. If the clay layer is very wet or just wet, the "fracturing" does not occur very well. If it is "dry" clay, better fracturing occurs allowing water to pass ....
I'll be quiet now.
P.S. Weight down the front and rear ends on your 790 -- it's not a very heavy tractor. If MFWD is on your tractor, use it.
When you use the subsoiler, make sure the "foot" is at an angle -- down -- about 10 to 15 degrees. The mast of the implement will "lean forward" slightly, foot back and head forward -- not straight up and down. Not enough of an angle, the implement will start "floating" and work its way up in the soil; too much of an angle, and -- well -- you get a "brake" or "break."
I used a ripper tooth off a D9 CAT similar to that to bury some underground irrigation pipe. To get it to stay in the ground, I had to drop the tooth in a hole, and the run a chain from the top link to my tow bar to stop the 3pt from rising. Worked really well, but stopped my 2520 with a bucket full of wet clay a few times (traction limited)...
I welded a chain link on the bottom of my tooth and used it to pull some black irrigation pipe. Then, after I was finished, ran the electric wires inside the irrigation pipe. You have to use the right electric wires because irrigation pipe isn't condiut, but it is cheap and beats digging a trench by hand.