A static EQ cut is always there. If a singer is boomy on three notes out of thirty, a fixed cut at 220 Hz takes the weight out of the other twenty-seven as well. A dynamic band applies the cut only while the level inside it is over the line.
Direction
Cut over threshold works like a compressor for one band: nothing happens until the band crosses the threshold, and past it the gain falls.
Boost under threshold works the other way. The band lifts as the level falls below the threshold, which fills in a part of the spectrum that only goes missing sometimes.
Threshold, ratio, range
Threshold is the level in the band that starts the action, measured by a filter tuned to the same frequency and width as the band itself.
Ratio decides how much of the distance past the threshold turns into gain change. At 4:1 the band moves 3 dB for every 4 dB the level crosses by. At 1:1 nothing moves at all.
Range caps the total. Set it to 6 dB and the band can never move further than 6 dB however loud the source gets, which is what keeps a loud passage from being scooped hollow.
Attack and release
Attack is how fast the gain moves toward the amount being asked for, release how fast it comes back. Both act on the gain change itself, so moving the threshold arrives over the release time instead of stepping.
Fast attacks near 5 ms catch a consonant or a pick attack. Slower ones near 40 ms let the transient through and act on the sustain behind it. Releases under about 80 ms on a low band can follow the waveform closely enough to modulate it, which sounds like a wobble rather than a level change.
Shape and width
A bell is placed by frequency and width. Both shelves use their frequency as a crossover and ignore Q, so a high shelf at 8 kHz set to cut over threshold acts as a broad de-harsher over everything above it.
The detector listens through the same shape it applies: a band-pass for a bell, a low-pass or high-pass for the shelves. One gain is calculated from the loudest channel and applied to every channel, so a band that ducks does not pull the stereo image toward one side.
Example: boxy 300 Hz on a spoken take
Bell, 300 Hz, Q 1.6, cut over threshold, ratio 3:1, range 5 dB, attack 10 ms, release 180 ms. Lower the threshold until the harshest sentence pulls the band down and the rest of the take leaves it alone.
How the band is applied
The band is one all-pass section mixed against the dry signal, a structure whose coefficients depend on frequency and width but not on gain. That matters here: the gain changes every sample, and rebuilding a standard EQ section 48,000 times a second would cost more than the rest of the chain together. Mixed this way the response is exactly the requested gain at the centre frequency and exactly unity at DC and Nyquist, whatever the gain is doing.