I've posted this explanation in other threads, so I'll just cut and paste it here.
I have had seats come in that seemed to be tightly installed because latch and the seatbelt were both used, but in reality, neither system was restraining the seat at all. An example, (contains reason why most cars do not allow you to borrow latch anchors from outboard to use in center.)
A father came in with a Century Smart V infant seat in a Ford Explorer. It was installed with latch (add on kit) in the center using outboard anchors. The seat belt was also routed through the seat also, however, the way the belt sat on the edge of the shell, it was in non-locking mode.
Issue with latch: There are three basic ways that latch anchors are installed in cars.
The most common way is that two bars, each containing 2 latch anchors are bolted to the vehicle or seat frame on each of the outboard positions. The bar that the latch anchors are attached to is actually about 10 inches longer than the spacing of the latch anchors. Most cars do not have the spacing to install 3 of these side by side. If you were to use the inboard anchors for each bar to install a seat in the center, the direction the car seat would be pulling on the anchor during a crash would be counter to the direction that the bolt is intended for and would strip the threads leaving the car seat unrestrained.
The second most common way, and the way that is found in most cars that do allow latch use in the center, is that a long bar containing 4-6 latch anchors is welded to the vehicle or seat frame. Since the system is welded rather than bolted, the direction of the pull on the anchor is no longer an issue. Note, if you can flip one section of the seat forward you do not have this system.
Third way latch is installed, but by far the least common, is that latch anchors are built into the seat of the vehicle. As in each anchor individually welded or bolted on.
Note: you have no way of knowing FOR SURE what type of system you have unless the manufacturer tells you. FOLLOW DIRECTIONS AT ALL TIMES.
In the case of the Explorer I am talking about, we called the manufacturer and found out that they use the same latch equipiment as Saab, which I know is the first kind, because that is who I got the latch anchors for my demo bench from.
Therefore his car seat was not really installed with lath at all.
Problem with the seat belt:
The seatbelt sat at an unlocked angle against the car seat base. The female end seatbelt could not be twisted or shortened with a belt shortening clip as it was encased with a plastic sleeve.
Therefore this rock solid car seat was not actually installed in this car at all.
This particular example is not the only time I have seen this kind of thing happen, it just sticks in my mind because it was the first time I saw it.
HTH,
Kimberly