Question: Sorry if I haven't worded the title very well, feel free to improve.

Let me explain before I ask my question...

We have purchased a [car carrier](https://www.petbarn.com.au/playmate-airline-spec-open-top-pet-carrier-white-grey-medium) (small version) for our dog and also purchased [some straps](https://rads.stackoverflow.com/amzn/click/com/B005SH05E6) to secure it to the seat; however we have noticed that even though it is secured to the seatbelt it is still a little loose (you can move the carrier forward a bit before it attempts to prevent further movement).

I have heard someone else that has used these same straps state they have instead put the straps around the whole back of the seat rather than use the seatbelt and they have stated it is now much more secure/tighter against the seat and hence prevents much movement.

So my question is - which method is better/safer?

* Securing it around the seat, to me, *seems* like it may be a better option as it helps prevent movement, but I'm wondering, because of this lack of movement, would our dog now have to sustain more **g-force**?
* Securing it to the seatbelt, to me, would seem to propel the carrier forward on an impact until it is stopped and then our dog would be thrown forward into the front of the carrier in a more "jerky" fashion then when secure but *may* not have to sustain as much g-force!?

As a side note, if it matters, we have been told by [CPS](https://www.centerforpetsafety.org/) that the straps we purchased are **not** *strength rated*; which I assume means they have the potential to break at high points of impact, but we are open to purchasing better ones if need be, but I am also cautious about using straps not specifically made for carriers as I have heard using seatbelts around carriers [can potentially crush them](https://www.youtube.com/watch?v=YnES6NIMHsM).

Answer: 
When your car drives head-on into an unyielding obstacle, the front end of the car will absorb energy by "crumpling." The relatively rigid passenger compartment will take somewhat more time to come to a stop than the front bumper will take, and so the passenger compartment *and anything that is rigidly attached to the passenger compartment* will experience a lower peak acceleration than the front bumper will experience.

Instead of a car, Let's imagine a bus that runs head on into a wall while traveling at 60 miles per hour, with your dog on the back seat. If your dog is not strapped in, then as the bus starts to crumple, your dog will continue at 60 miles per hour, flying through the air, until it impacts the front of the bus, which likely will have come to a compete stop by that time. But, if your dog is strapped in to the back seat, then it, and the back seat, will slow down more gently while the front of the bus is crumpling.

The difference might not be as great in a shorter car, but I'd still rather be strapped in than not.

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P.S., When the police officer looked at me, sitting unhurt on the grass next to the wreck of my car he said, "I don't need to ask whether you were wearing your seat belt." I guess he'd seen enough people who were and enough who weren't to be able to tell the difference.
I refer you Bob D's answer above for the physics.

At the moment of impact, the carrier will move forward until it reaches the limit of the restraining straps. The straps then undergo a deformation (elastic and inelastic), over the scale of a few centimetres, that decelerates the load. This process is the same whether the straps are loose or tight because the load is moving at the same speed at the moment when they reach their limit and begin to bite.

So it doesn't really matter if the straps are very tight or not. As long as they not so slack that the carrier can move forward enough to impact something in front.