Mayby some official word about what Pietia wrote - becouse I am very confused (and my friends too) 
I wouldn't hold my breath. Best to just sit down with your friends and make up a house rule that satisfies all.
I don't know if it's the correct way or not, but we have played TS like this:
a squad member wishes to fire at a target that is not it's priority target:
-a Tactical Sense roll is made (by a squad leader, should one exist; or the model itself if it has the skill)
-if it succeeds, the model asking to switch it's target is allowed to do that for the rest of it's turn
-if it does not succeed, the model asking to switch it's target can't do that and must target it's closest enemy
-only one TS roll can be made per turn per model (ie. if there is a Colonel leading a squad with a Captain AND a Sergeant and each have TS, you can't make 3 rolls to try to switch targets) and should it succeed, it holds for the rest of the model's turn and if it doesn't succeed, the model must use all it's remaining shooting attacks to attack it's closest enemy.
Then another squad member wishes to use TS and the process repeats itself.
We've used TS like this without Comm Helm/Comm Links, but then again the reason to actually USE Comm Helm/Comm Link is a bit vague if it's not needed to convey Tactical Sense "orders"... I don't know if it's needed to use the skill or not. The TS skill description is incomplete and too ambigious so there's room for many interpretations (I'm sure we've played "the wrong" way, but it does work decent enough in our games).