its not exactly something that can be directly proven. But the gist of the theory is this

every time there was an uprising, take Czech for example, USSR would send in the tanks as a show of force, and it was effective. Then Tiannem square happenend, and this photo became immortal.

In the years that followed there was a surge in protests, and instead of fighting tanks and throwing molotevs, people would walk up to the them unarmed, asking them to shoot them in cold blood. this can be seen in all across eastern europe and in moscow itself. The tankers wouldnt shoot unarmed civvies and the threat of the monster that is the state vanished overnight.

of course its not as simple as that, and gorbachev had his part to play but its an interesting theory.

i think i heard it presented in a tank documentary on netflix (its about tanks, and there are no tranny pedo tanks, so they couldnt pull their usual shit so its pretty good)