Basically, if you attack enemy towers and there are no creeps around, it's considered backdooring. There are a few rules of exemption, such as creeps are literally right behind you and you decided to get hurt yourself for some reason, the tower is in deny-able range(10% hp), or all the creeps around you die while hitting the tower. Whether or not heroes are alive don't matter.
Basically, imagine at the start, Flan gets four of a kind, and sends three off to hurt the tower before any creeps can come. That is backdooring. Imagine Flan has been pushed to her tower, and covertly sends three clones to hurt the tower, that's backdooring. Imagine that your being pushed inside your base, and Yukari teleports to the other persons base and starts wrecking stuff. That is backdooring. It's all about the creeps man.