It's actually very simple. When CW ends after 12 rounds, You hear the win/lose sound. Wait for 5 seconds and leave the room. Does not work everytime. Only after 12 rounds, Not halfway through. The BP Bugs gives you 20k to 80k BP at one goal.
I'm lucky. I'm get 50 k at 1 times then Clan War (CW) again and get 40k. Now my BP is almost in the LIMITED. xD