Peak detection in Performous code

Question

I was looking to implement voice pitch detection in iphone using HPS method. But the detected tones are not very accurate. Performous does a decent job of pitch detection.

I looked through the code but i did not fully get the theory behind the calculations. They use FFT and find the peaks. But the part where they use the phase of FFT output, got me confused.I figure they use some heuristics for voice frequencies.

So,Could anyone please explain the algorithm used in Performous to detect pitch?