The strap is only used in the Deep Sleep Modes, typically this is on a long haul flight.
These flights almost always have a winged headrest device in the seat. The FaceCradle straps are attached to this device and do not go around the back of the seat. Please see the video on our page that shows how to attach these straps to such device. Where this device is not present, usually it is a budget or domestic airline and most often there is also no TV screen.
Some airlines however, do have an odd configuration of no device but with a screen. In this case if you mention to the person behind that you will not recline your seat to sleep because you have a FaceCradle, they will be very pleased. You would then ask them if by chance your strap happens to drop over their screen, would they mind adjusting it for you. If you feel uncomfortable about asking this, then you are still able to use 3 of the 5 modes of your FaceCradle to get to sleep.