This scene is driven by a simplified collision code. The buttons above correspond to the first 6 bits of the collision masks of the red and blue plates and the collision check masks of the green spheres. Press the buttons to toggle the binary values. You can read the base 10 value next to the binary representation. This would be the number you would use for the collision mask parameter of the WTGroup::setCollisionMask() and WTGroup::checkCollision() methods. You can play with the binary values to get the effect you want - collision with both plates, only one plate, or neither plate.
The simple way to think about how masks work is that if one or more pairs of 1's line up (reside at the same bit location) between the collision check mask and the mask of the container that you will hit, then a collision is detected and a WTCollisionInfo object is returned from the checkCollision() method. If every bit location is such that either a pair of 0's line up or a 1 and a 0 line up, then no collision is detected.
Another way to think about it is to consider 0 as FALSE and 1 as TRUE. Between the collision check mask and the mask of the object you will hit, each bit location is "anded." If any TRUE's are returned, then a collision is detected and a WTCollisionInfo object is returned from the checkCollision() method.
WTContainer::setCollisionMask, WTContainer::checkCollision