dynamic equalization at mastering songsWhen composing electronic music, sound producers and audio engineers often use dynamic sound processing devices: compressors, gates, multiband devices and so on. Today the use of processors for dynamic equalization is not uncommon as well. Dynamic equalization includes simultaneous use of equalizer and compressor functions.
In the process of usual equalization when hip hop mixing and mastering you pick out the desired frequency, adjust the bandwidth and necessary reinforcement or attenuation. Therefore, you process only a certain frequency range of the exact value in dB. In the process of dynamic equalization reinforcement and attenuation of the selected range are not the same within different periods of time. It depends on the signal level of the picked out frequency band at a particular moment. Dynamic equalizers are adjusted in such a way that the signal of certain frequency will be attenuating only in case its level exceeds the specified threshold. The higher the signal level of the selected frequency, the stronger its attenuation.
It can therefore be concluded that dynamic equalizers are capable of balancing the signal level of a certain frequency band. Everything is also true for boosting the selected frequency band. However, you should take into account distinctive features of different plugins.
For example, during the exceedence of the threshold the signal in Voxengo GlissEQ equalizer is strengthening whereas the signal in TDR Nova plugin is reducing.

dynamic equalization at mastering tracks onlineDynamic equalization is usually used in the following cases:

Removal of resonances.
Elimination of unnecessary side tones.
Dynamic equalization.

Dynamic equalizers are used for the removal of resonances on harmonic instruments. If you use usual equalization, the removal of resonances on a certain chord can lead to the removal of useful frequencies on another one. To avoid it, the resonance is removed dynamically. The removal of the picked out frequency happens if the level exceeds the specified cutoff point.

Besides, dynamic equalizers are quite often used during backing vocals mixing to reduce hissing and droning sounds. As stated before, dynamic equalizers can be used for dynamic instrument equalization (on the analogy of multiband compressors).

Some of the best known dynamic equalizers are the following:

iZotope Ozone 7;
Brainworx bx_DynEQ v.2;
Voxengo GlissEQ.

Out of free of charge plugins I have found TDR Nova equalizer to my taste.