Don't know how to hook it up!

The swivel in the top link is sometimes used on 3 pt implements to allow the rear of said item to move over uneven terrain "disconnected" from the tractor. If you didn't have this, if the tractor went up a hill, the rear of the implement would (theoretically could) bury in the ground and the tractor's rear wheels would come off the ground. Obviously something would bend or break. Tractor front going down (as compared to up) is not so much a problem as the rear of the implement could be lifted into the air without hurting anything....course you wouldn't be plowing either.

For discing, or plowing (in N. Tx we use discs for plows in heavy black clay) the discs are usually set in a Vee with the angle of the Vee determined by what yo want to do and how fast you want to run. The front row usually pushes the soil out and the rear row pushes it back in place, supposedly nice and loose.

Some 3pt implements have a swivel rear section that can be detatched, or rolled over the front for added weight when trying to "crack" hard soil.

My 2c

txblu