Quenched flow is an assay technique used to study a reaction at defined time periods. This may be because it is not possible to quantify the compound of interest in real-time.
This type of assay requires the use of specialist equipment, a quench flow, to quickly mix the reactants (to start the reaction) and then, at a pre-defined time interval, add the quenching agent to stop the reaction.
Temperature can be used as a quencher where heat labile proteins are being used, changing the structure of the enzyme and thus terminating the reaction. Freezing reactions is also possible, however it may be difficult to separate out components when this is done (as everything is in solid form).