the general consensus seems to be that it is simply an autotune function with the tuning delay, which allows the singer's voice to naturally slide into the pitch, set to 0. When set in the proper key, his autotune setting will not allow his voice to become flat or sharp and his voice simply jumps from one diatonic pitch to the next creating the fluttery, in-human effect you hear on record.

It's the same effect used on Cher's "Believe."