Segmenting is a common technique used to separate a grapefruit or orange from it's peel and pith. to segment a grapefruit (or orange):
Slice a thin piece of peel from both the top and bottom so the flesh of is exposed. This will give you a flat surface on both the top and bottom of the grapefruit.
Following the contour and shape of the grapefruit and slice the peel and pith away from the grapefruit. Use a butcher’s knife and move from top to bottom, ensuring that your grapefruit is resting flatly on the cutting board.
Continue this process, turn the grapefruit after each slice until the peel and pith have been removed.
Using a paring knife, make cuts along the inside of the membrane into the center of the fruit. Repeat this process along the membrane found on the other side of the segment. Once you’re knife has reached the center of the segment of the grapefruit will pop free. Repeat this process until you have removed all of the segments from the grapefruit.