This article is not 100% accurate, as I could not find the required information. Therefore, use this only as a general concept.
Backdoor protection is a structure buff that protects the structure from being backdoored, or ganking a structure, and destroyed before the towers in front are destroyed. It makes towers unable to take damage (or gain armor, not verified). When lane minions are present, the protections are removed. The core gains around 500 armor (a total of 580) to prevent backdoors without minions, and won't take any damage if all three inhibitors are still up.