Re: What does the top tether do?
Don't go hide, if you're asking the question, chances are there is a lurker or two with the same question. All of us had to ask questions like this to get educated about car seats in the first place.
For a forward facing seat, the top tether adds another point of attachment for the car seat. It holds the top of the seat back and adds stability. What this does for the child is reduce head excursion (the distance the child's head travels forward) by 4-6 inches. In Canada, top tethering is required by law for all forward facing child seats. Best practice dictates that all forward facing seats be tethered. The top tether is part of the LATCH system. It's the T in the acronym. (Lower Anchors and Tethers for CHildren).
For a rear facing seat, the top tether adds stability and it reduces rebound. In the US, only Diono, Britax and Combi allow for rear facing tethering. These manufacturers include a d-ring with their seats to create a tether point with. There is debate about the benefits of rear facing tethering in general and in respect to using it with newborns and small babies. Britax is the only manufacturer that allows Aussie tethering (attaching the tether to the tether anchor over the top of the seat when the seat is rear facing) but that is generally not used as it is inconvenient to get the child in and out. Most of the rear facing tethering done is Swedish style. Most of the Swedish ERF seats use tethers. One of the reasons why those seats are not available in the US is that in order to pass testing, the seat cannot rely on tethers, foot props, or bracing in order to pass.
CPST-I and SAHM to DS