Of the bang that you'ld like is like you can see in this clip at 0.48 minutes, than here's my idea on some things.
1. Why start from a down position? The down position is an extra condition to do the bang in the dog's mind. You have learned him this chain, so smart doggy
2. Like I have said a few times on this forum, it's all about muscle memory. I prefer to lure him in a bow position (don't ask him !!) and then lure his head in a twisting motion. When he turns his head you'll see that at a certain point he starts shifting his weight to the opposit side (the body follows the head theory). Mark the weight shifting and keep increasing, so eventually he falls over. THAT'S the start of the end behaviour being falling down like you want to. Start working on higher tempo and round up by teaching him to keep his head on the ground. Keep in mind that your dog has a prefered side to lie on. When he's in a down, check out if he shifts his weight in a certain direction. If so, turn the head away from that side.
3. I found that once he/she knows the behaviour it's best to ask it after a well known and selfrewarding behaviour to get a higher drive. Eventually the new behaviour "bang" also becomes self rewarding.
Hope this helps you out a bit and feel free to ask.
Language faults are possible since I'm Belgian