Tokøta from Mastering.com shows how spectral synthesis works in Logic Pro's Alchemy, using a spectrogram to resynthesize an imported sample from scratch.
After importing a sample in spectral mode, Alchemy displays its frequency content as a spectrogram, low end at the bottom, harmonics rising above.
Two resynthesis modes shape the result: noise resynthesis creates unpitched, textural sound effects, while pitch resynthesis produces more melodic, harmonic material.
Controls like length, variance, gate, and bandpass let you blend and filter the resynthesized signal, and the resynthesis volume has to stay on for the sample to play at all.