I know, another duckdiving thread, foot placement Q

Now I've been surfing for a while and I just moved down to a 6'4 shortboard. The transition wasn't so bad, I'm actually progressing nicely, except for my duckdive. My board is 6'4 x 19 x 2 3/8, I'm 6'0 and weigh 170 pounds, so it should be fairly easy.
I can submerge the nose fine, but after that things fall apart. I know my backfoot is supposed to counteract the front of my board diving down. The thing is I can't do it properly. Am I supposed to kick my tail quickly to sink the board? Or apply pressure with my foot and keep it on the tail? Any time I attempt to keep my foot on my tail I feel really akward and lose my balance. Any help would be appreciated.
I can submerge the nose fine, but after that things fall apart. I know my backfoot is supposed to counteract the front of my board diving down. The thing is I can't do it properly. Am I supposed to kick my tail quickly to sink the board? Or apply pressure with my foot and keep it on the tail? Any time I attempt to keep my foot on my tail I feel really akward and lose my balance. Any help would be appreciated.