The purpose of Flash Boom is simple. It will give you an estimate of how far away lightning is. The rule of thumb says to allow about 5 seconds for each mile (3 seconds for each kilometer). In reality it is more like 4.72 seconds per mile and 2.99 seconds per kilometer. For more details, please visit http://www.wikihow.com/Calculate-the-Distance-from-Lightning
Tap the FLASH button when you see the lightning. The button will change to BOOM. Tap the BOOM button when you hear the thunder. Thunder will usually not travel more than 15 miles (24 kilometers).
If you are less than 5 miles from the lightning, please take cover.
The original Flash Boom program I knew of was writen by Kim Moelgaard for the Palm OS. I adapted the concept (without access to the code) to the iPhone.