I dont usually post here but having researched as much on the bulgarian methods and watched some of the seminars I'll elaborate on some of it.
Ivan started with using many exercises and typically did the Russian system wheer they lifted moderate loads for very high volume, and a lot of variety. What he found was that they got better at doing those movements at that level of intensity, so they could endlessly do 80% weights for 10 doubles or something with no trouble, and technical proficiency.
Once it came to comp time, their results marginally improved if at all. He noticed that the effect of a comp environment on the human body was very different to a training environment, in a sense that you are lifting to max and that adrenaline (a hormone precursor to "fight or flight") was more evident during comp time.
So he started increasing the intensity and reducing the volume, by taking away exercises that did not provide direct benefit/relation to the competition movements. As the lifters adapted to the intensity the volume increased for the lifts that provided direct benefit and the other lifts were removed.
Eventually they were doing 3 movements, snatch, clean and jerk, front squat. They lifted to a daily max with varying volume, (NOTE: daily max, meaning the max on that day). Typically they weer allowed a certain numbver of misses on the movement, ideally as olympic movements are very dependent on technique and a miss does not usually mean "not strong enough" like in powerlifting.
the science behind Ivan's approach was that he believes the CNS would adapt to imposed demands but more or less that it was the regulator. As such it protected you from causing harm, ie squatting 200 one day and the next squatting 160 due to systemtic fatigue or any other sort of ailment.
At the same time he believed that if enough stress is placed on the organism you could promote muscle hyperplasia, increasing the number and density of the cells via splitting.
They trained year round, no deloads, no breaks. They ate whatever they wanted, they were on drugs that was a given but the mind and body is very powerful. There is a seminar on youtube you can watch which is very detailed.
In my own experiments, I adopted the same mentality for one lift, my squat. I have been stuck on a 180kg squat for the better part of a year.
Slowly working my frequency and intensity up, I worked up for the past 3 months to squatting to max 5-6 days a week, alternating squat/front squat and their paused variants.
I took my squat from 180-200 in that space, and more is still to come. Mobility has improved and aches and pains that used to be present every day are now gone.
Other good sources of info from this are interviews on John broz who was coached by Antonion Krastev, Ivan Abadjiev's super heavy lifter.