Frequency modulation is an example of phase modulation, and which we covered earlier.
An osc~ object is the same as phasor~ object connected to a cos~ object. The phasor~ provides the phase, 0 to 1 in Pure Data (corresponding 0 to 2 pi in the real world).
Here, we use a sqrt~ object to change the phase. This leads to audio with a large frequency bandwidth. With phase modulation, it is possible to get complicated sounds with only a few changes.
By looking at the audio waveform we can see the low-frequency parts (slowly changing) and high-frequency parts (changing quickly). We could confirm by plotting Spectrum from Analysis menu in Audacity.