I have ran a head restaint strap on my helmet for years. Have a bulging disc in my neck that is not quite bad enuf for them to cut on. (Yet, but i love racing) anyway. You can't drive to your best ability if your head is laying on the engine. (I have gopro vids from indoor concrete race to prove this)
Also, I have researched these straps extensively throughout the years and I have never heard of any safety issues with the use of one, when they are installed in the correct location, and adjusted properly.
I can send you pics of exactly where mine is located if you need me to. I run a K-1 sport in the UAS. the location of the d ring on the helmet is very important!! Next, the driver has to learn to adjust the strap correctly!! it takes some getting used to. And ,yes, I will adjust mine while racing. you have to make it comfortable for the driver and functional both. send me a pm if you would like pics.
Matt#42 OVUAS