Coman tie-breaks are just like regular tie-breaks, except the side changes are altered to switch after 1, then 5, then 9, then 13, etc. points.
The rationale is that it allows each player to continue serving from the same side they have been during the set. It also avoids someone having to serve the point at say 2-3, then switch sides before serving from the other side at 3-3. I believe "fairness" is also cited as a reason as by changing sides more frequently, the effects of sun/wind and there being a good/bad side have less chance of influencing the score since players are switching sides more often and the streak of points on the good/bad side are shorter.
I'm personally not a big fan of them, but that is the rationale, and some sections call for them to be used, at least come districts and sectionals. I think Nationals calls for them to be used too.