I work for a supplier of seatbelts, airbags (all types), steering wheels, and night vision. Actually I work in the test lab for these things. rbattelle is right. I will try to go as much into detail without giving away proprietary information of my company.
It is a function of the seatbelt retractor called ALR. (Automatic Locking Retractor). It is activated by fully extracting or pulling out the seatbelt. Once the ALR mechanism has been triggered, you will probably be able to hear a very faint clicking sound as you feed the webbing back in. I am not sure where the retractor is mounted in the Ram (most likely near the floor of the B-Pillar), but if you put your ear by the bottom of the seat/B pillar you will probably be able to hear it. ALR should only engage when the belt is pulled out all the way. If it engages early, it is a nuisance. It should unlock a little before the at rest position when not in use. Either of these nuisance conditions can be caused by a misalignment in the timing mechanism. It often helps with the installation of child/baby seats. Generally ALR will be found everywhere except the drivers seat. I have heard that maybe the Corvette has them in the drivers seat to help keep the driver in place. To basically lock you into place. Don't quote me on the Corvette line tho. There are a few other features of a retractor that you may run into.
Another mechanism senses the acceleration of webbing as it is being pulled out. Ever hop in (in a hurry) and try to put your seatbelt on (too fast), only for it to lock, and have to feed it back in before being able to put it on. Think of what your body does when you are in an accident. You are not bolted to your seat and so you will want to move/slide forward. When you accelerate forward, the seatbelt webbing also accelerates. The retractor senses this and locks. This is called websense.
There is also the ELR function. Emergency Locking Retractor. Think of a basketball on a shot glass. Grab the shot glass and start sliding it across the bar. Get going at a good clip and suddenly stop the shot glass. The basketball keeps moving. This "basketball" hits something else and triggers a locking mechanism. Think of you stopping the shot glass as the vehicle striking an object in a crash. This will lock the webbing as well. If you really want to try this in your vehicle, go somewhere where there is no traffic. Slam on the brakes on an asphalt or concrete road. Same effect. I take no responsibility if you get rear ended.
Then there is tilt lock. If you roll your vehicle, you will find out first hand how this works. Simply put, the vehicle can be on an angle to a certain extent and the seatbelt function as if it were on a flat road. After that it will lock. Jumpin' on it on a 4-leaf clover e-way on ramp can lock a retractor too. This condition amplifies the combined effect of the previously mentioned ELR (g forces acting) and the car tilting/leaning.
Then there is the pyrotechnic pretensioning system. To put it really bluntly, it takes the slack out of the seatbelt system. There are also yielding components to lessen the forces on the body. All of these components of a retractor must meet strict FMVSS and manufacturer standards and function over the lifetime of a vehicle. As much as you want your wife in her seat as to not mess with the radio, if you are in any doubt, take it to a dealer. Whew I'm winded. Sorry for the long post.
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