Adjust the relative amount of the attack transients
The difference between the envelope with the fast attack time and the envelope with the slow attack time yields a control signal, which has significant amplitudes only when the incoming signal is in some "transient" state.
This is because the difference between the envelope-followers with fast and slow attack tends to zero in a sustained phase. Multiplying this control signal with the incoming signal yields a signal, which contains the transient component.
The extraction of the release component is similar: it is done with two envelope followers with identical attack times a different release times. The difference between the envelope with the slow release time and the envelope with the fast release time has significant amplitudes for decaying signals.
Multiplied with the incoming signal, we have extracted the release component. Now we can adjust the relative amplitudes of these two signal components seperately.