Nelsonite is a wood stabilizer, by impregnating the scales with it I discourage warping and other wood related problems from developing down the road ...

Introducing a vaccum pulls the stabilizer deeper into the wood.

There are several ways to create your vaccum, the easiest way is to use a large mason jar, heat your stabilizer in the jar, a pot of very hot water works well for this...
Seal the jar and allow it to cool.
The cooling creates low pressure inside the jar and forces the nelsonite deep into the wood.
I know it sounds wrong, low pressure in the jar should pull the oils out of the wood, but somehow it does work... I think it is a rebound effect. The vaccum opens the pores of the wood and allows the nelsonite in deeper, and as the vaccum is released, the pores close and hold the stabilizer better.
I have seen threads on this before, maybe someone else can explain it better.

There are also ways to create vaccum chambers if you look around...