Question about rear tethering

I have a question about tethering a rear facing Diono Radian RXT in a Prius.

I understand how to install it and where and will have it checked but is it safer? Does it provide more support or slam the child's head back into the seat? The child is 11 months.

Also when installing the tether does it need to be tight with no movement or just so there is no slack?

There's a thorough technical encyclopedia entry on rear facing tethering at Safety Belt Safe USA's site at http://carseat.org/Technical/tech_update.htm#toptetherRF indicating there's some potential benefit from reducing rebound and reduced head excursion in some types of crashes. Just remove the slack from the tether strap if you tether your Radian rear facing, the tether strap doesn't need to be so tight that there is no movement (your installation should have less than 1 inch of movement at the Radian's belt path before you tether it rear facing, though), nor so tight than you can play a high note on it like twanging a guitar string. Avoiding overtightening the top tether strap is particularly important if you ever happen to tether a rear facing Britax convertible carseat because overtightening the tether RF has been known to damage the recline mechanism on Britax convertibles.

Thanks for that website it was really helpful but I still have a few questions: 1 - does it matter which side of the passenger seat I tether the CS to? I am worried that if I tether it to the left side it will rotate the CS towards the window in a crash? and 2 - the website mentioned that bracing the CS against the front seat is helpful - I thought that was bad and could brake the seat? also is it supposed to be braced or just resting against?
Thanks sooo much I have never used a rear facing seat that isn't and infant carrier before and am trying to learn as much as I can before I put the child into it

In terms of crash statistics, the passenger side and the driver's side are about equal in terms of side impact collision frequencies, FWIW, and a center installation is technically safer if possible to install there because it's farther away from either side of the vehicle. I personally prefer to tether rear facing to the innermost front passenger seat leg when possible if I'm installing behind the front passenger seat because it's closer to the center. I also prefer the innermost front passenger seat leg/rail rather than the front driver's seat leg/rail for a center installation because of the likelihood of the driver's seat needing to be moved on its tracks if the car is shared by drivers of varying heights. It's also good to bear in mind that rear facing tethering is optional for the convertible carseat models that allow it (Radians, Britax convertibles, and the Combi Cocorro) rather than required, and rear facing in and of itself offers added crash protection.

Bracing against the front seatback isn't allowed by every carseat manufacturer, but it is allowed by the Radian and by Britax convertibles, but it also has to be allowable by the vehicle manufacturer, too. It's probably not allowed to brace nor touch the front seatbacks with a rear facing carseat in your Prius if the car is equipped with advanced front airbags due to possible interference with the sensors for the advanced front airbags. If your Prius is equipped with advanced front airbags, then there's likely a warning blurb in the car's owner's manual (in the airbag section and/or the carseat related sections of the owner's manual) stating not to allow a rear facing carseat to touch the front seatbacks due to potential interference with the advanced airbag sensors.

Thanks a bunch - again!
I actually had to call Toyota because the manual didn't say one way or another about bracing. They ended up telling me that as long as it was touching and not bracing the CS they didn't think it would affect the air bags but I should check with the Highway Patrol or Fire Dept. to be sure - which I was planning on anyways.

I noticed from one of your other threads that you have a 2011 Prius (so do I ) and there definitely is a warning against touching/bracing against the front seatbacks in the vehicle owner's manual due to the advanced airbags. I'm going to link that other thread to this one to be sure you see this because it's pretty important info. The warning blurb in the owner's manual is easy to overlook but it's in there. You'd think Toyota would make it very visible and easy to spot but instead it's kind of buried, and even the customer service rep you talked to at Toyota apparently isn't aware of it. Here's a quote from the 2011 Prius' online owner's manual's airbag section on page 20 of the .pdf file (numbered page 133 of the printout/hard copy of the manual) :

Child restraint systems installed on the rear seat should not contact the front seatbacks.

Click to expand...

If you have difficulty in avoiding touching the front seatback(s) with your rear facing Radian, there is a rear facing angle adjuster available for the Radian to help achieve a more upright rear facing recline angle.

wow! I am not sure how I missed that - I read EVERY section of the air bag section and the car seat section which starts at 134 I believe - lol. Thank you soooo much for posting that. I am no sorely disappointed in Toyota's customer service number . I don't have the Radian yet (ordered it 2 days ago) but I also have the angle adjuster coming but it says it may take a month! so I just wanted to be sure in case the seat didn't fit without it.
What car seat do you use in your Prius?

My kids are almost 7 years old, and 10 years old and they usually use belt positioning boosters (Britax Parkway, Harmony LiteRider) in our Prius. My oldest passes the 5-step test to forgo using a belt positioning booster when she rides in the center rear seating position and fits better side by side using the vehicle seatbelt alone while riding next to her booster riding little sister there. We got the Prius about a month ago and really like it so far. 53.3 mpg on our most recent mostly highway driving road trip over spring break.