BACKDOORING

Backdooring: You are not allowed to attack any tower on the map without creepsupport.

1. You have to enter the base with creeps but they dont have to be in the same line as you.
2. As soon as creeps enters the base, the complete base is allowed to be destroyed.
3. As long as there were creeps in base when you entered, you are allowed to keep attacking even if they die.
4. If you leave the base at this moment you need to have new creeps in the next attack. If a tower is denyable (below 10% HP) and is not a base tower, backdooring rules do not apply.

DEFINITIONS :

Creeps "Enter the base" when they are upslope on their way into the base.
Creepsupport : The creeps are within 600 Range

see at no 1, it means if you got mid rax,you only have to push mid until creeps got inside base and then straight to bottom rax.this is not call BD , this is FRONTDOOR

p/s : people who backdooring in public is emotional and desperate person. no pic here,find for yourself

i copy from this link,lazy to write.
http://www.dota-board.com/dota-league/single-instant-games/346-update-backdoor-rules-denyable-towers/