You use the phone as a detonator. Call a number, phone rings, earth shattering KABOOM. Similar method is used in radio controlled commercial demolition and blasting. The number of common things you can find in your house and use as a remote detonator is pretty scary. No, I won't go into details.
That technique predates the GWOT and I kind of doubt that Bergdahl could have been the first person to show it to his captors. What is more likely is that Bergdahl gave information on routes, vehicle staging, vehicle and weapons load and distribution within convoys and other things.
In example, if you know that the standard separation for vehicles in a convoy is 100 meters in terrain type "x" but only 25 meters in terrain type "y", you can distribute your devices or plan your ambush more effectively. Same with rate of travel (speed of the guys or vehicles) or weapons distribution (is the .50 caliber machine gun on the first, middle or last vehicle).
Similar information for response and escalation in the event of an ambush would be extremely useful. If you know what the rules are for calling in artillery or air support and/or the time lag from first contact to air/arty being on site then you can hit a convoy and leave before your target can escalate or respond. In example, if it takes 30 minutes for a gun ship to be on-site, you know that you can hit the convoy, light it up for 20 minutes and then still clear the area before having to worry about death raining from the sky.
Not saying that Bergdahl gave that kind of information. Just saying that if some random dude from the other side walked into my area and wanted to help me I'd ask him for that kind of information.
Thanks!