EQI stands for Eccentric Quasi Isometrics
It's a little known technique that is effective for working the muscles hard at the end of a set.
It can also produce quite a lot of soreness
So enjoy that...
The main thing about EQI's is that you fight to hold each position for as long as you can whilst very slowly fighting gravity.
Also be very careful of your form and safety on any exercises you chose to apply this to such as anything over head or keeping the spine in a good position.
This is my, mostly, Personal Trainer musings and information which I hope you'll find helpful!