Hydrolysis probe (Taqman) assays use a sequence-specific probe labeled with a fluorophore and a quencher. The figure below illustrates how hydrolysis probe assays work:
The hydrolysis probe assay uses the 5'-exonuclease activity of the Taq polymerase. When the hydrolysis probe is not hybridized, the 5'-fluorophore will be quenched by the 3'-quencher. The mechanism of quenching is Fluorescence Resonance Energy Transfer (FRET). However, when the probe is hybridized to the target during the combined anneal/elongation step, the dsDNA-specific 5'-3' exonuclease activity of the Taq polymerase cuts the hybridized probe. The fluorophore and quencher are no longer on the same molecule and the fluorophore will fluoresce.