Triangulation.

I've seen this term defined as: "the determination of the Y-chromosome DNA of a male ancestor by finding an exact match between direct paternal descendants of two sons of the ancestor."

That is, by searching through the traditional manner, establishing a paper trail to identify two of more lines of descent from different sons of a shared ancestor. DNA of the descendents can then be tested. If it matches this confirm a) their descent from this ancestor b) the ancestor's DNA. This early ancestor may be then be seen as the apex of the triangle. The next step is to expand the base of the triangle outwards, and the apex backwards. Using an example of this from the Blackstone DNA study:

Dan Blackstone and Bob Blackstone both believed themselves to be Gt Gt Gt Grandsons of Daniel Blackstone of Farmington, Maine, USA (1796 - 1868). He can be seen as the apex of this particular triangle. Dan and Bob the base. In Dan's case he traced his line back to Benjamin Edward Blackstone. Bob traced his line back to William Collins Blackstone both sons of Daniel. Their results on 25 markers have come back a 24:25 match scientifically confirming their paper trail. Ruling out a non-paternity event. However they differ on one marker DYS 447.  Dan is DYS 447/25, Bob DYS 447/24. This they will pass this on to their respective descendents.

Thus a mutation has taken place. A question is was Dan bn 1796 a DYS 447/25 or 24? Whose line has the mutation, Dan or Bob? If a descendent of a third son of Daniel was to be tested this may be determined. 

Any individuals to be tested in the future finding themselves DYS 447/25 will know they are descended from Dan's branch of the family. This could help them focus their research.

The aim now is to broaden the base of the triangle. That is looking for a possible predecessor to Daniel bn 1796. 

The question for them now is who are Daniel's parents? There are two candidates. 1) William & Elizabeth, the most likely parents and 2) William & Lucrietta (nee Blunt). This 2nd William is the grandson of William Blackstone and Abigail (nee Varney). 

It would be useful for them if a confirmed, male Blackstone, descendent from one of these lines was to take the DNA test. Someone not descended from Daniel. If they match, this shows a connection  but also one can be seen to be moving the apex back.

Any mutations evident but not common to Dan & Bob may help to focus future researchers on a specific family group, a specific son and a regional locality. Thus not only validating but guiding research.