What is the use of insert slot on a sound mixer.
An Insert slot, usually located on a channel strip is used for inserting special effects, such as an equaliser, compressor or other.
This usually occurs between line-in and board processing, but some boards may process HPF first
Use a TRS Y cable to insert special effects.
an insert is an access point built into the mixing console, allowing the audio engineer to add external line level devices into the signal flow between the microphone preamplifier and the mix bus. (https://en.wikipedia.org/wiki/Insert_(effects_processing))