Duck Diving bigger boards... pike?

[FidesMaris]

Been surfing a 6'4" Rusty Pirahna Surftech version, been having a hard time duckdiving it. I only weigh 115lb and the most I can sink that board is just submerge my head for a split sec. Then this other day I say a guy duck diving by just piking his legs straight up when the white water hits. Does it help sink the board more? OR should I find better techniques in duckdiving?

[Dec]

Push down deep with your hands, and using you foot, push even deeper down and the wave shouldn't push you.

It takes a time to emerge because of the water resitance against your board.

Rusty should be fairl easy to duck dive.