Grapefruit juice does NOT INCREASE ABSORBTION.
It reduces the ability for your body to metabolize certain drugs. In turn, the drug does not get broken down and it continues to circulate.
You can think of it as if the grapefruit juice is extending the halflife of the drug. It's very easy to accumulate TOO MUCH of a drug in your system in this way.
It has some application, but be careful and understand what you are doing. You are not simply increasing absorbtion.