How match detection system works? Well it goes through several filtering. To visually demonstrate that I turned off the elimination phase and let pieces stack up.

First find pairs of same color pieces then continue finding "junction" piece that connect other two piece so they became 'trio'. Now this 'trio' has a group tag unique to them to separate them from other "island" group. Whenever nearby piece with same color making "contact" with them that very piece became new member and forming 'four' which is minimum amount to make a match. 

It goes on like that by just adding new group member which is I called pseudo flood-fill operation.