Stencil jumping, the principle

From CFD-Wiki

Consider one element of a 2 dimensional mesh, for simplicity and consider a point O inside. The mesh element vertices are denoted by A,B,C and D and the vectors AB,BC,CD,DA,OA,OB,OC and OD are represented. The cross product of OA and AB will yield a vector perpendicular to the plane coming out of the monitor.We say that the cross product is postive.It will be observed that the cross products of OB and BC,OC and CD; and OD and DA are all positive.

This is not the case when the point is outside.

Image:/home/jaina/stencil quad1.jpg

Here we see that, all the cross products are not positive.This is the major testing criterion in the alogrithm.