Hello everyone. I am the developer of the app and will be happy to answer any tech question you have. That was a really challenging project.
Then the conversion itself. The goal is to type a number and one-two letters and get a relevant result. So there are many heuristics there. For example, 0.1 meter will be converted to 3.94 inches, 1 meter — to 3.28 ft and 10000 meters — to 6.21 miles. And there is much more of that.
At last we needed the app to work well on every device that is supported by iOS7. iPhone 4 is not a fast device by today standards. So optimization was a little bit challenging also.
Maybe you have any particular question in mind?