The number one rule of effective despotism? Crush the enemy before the election so that you don't have to crush them during the election. These are the rudiments. And Robert Mugabe is very, very good at the rudiments of despotism.
Zimbabwean elections have appeared to be relatively clean in recent years because Mugabe and his henchmen have made sure to terrorize, crush, hammer, and pulverize enough of the potential opposition so that the opposition simply does not show up. Thus on election day it appears that things flow smoothly only because those who might pose problems have been cowed into acquiescence. Make voting futile and futility follows.
The lesson that Mugabe and his people — those in the armed forces, those amongst the police, and those amidst the population of the self-proclaimed war veterans, very few of whom seem to have actually raised armes against Ian Smith's regime — seem to have drawn are clear: Force wins. Rule of law is determined by those who enforce the laws. The power of the state is the power to crush the political opposition.
In sum, scaring people works. It works in devastating ways and to undeniable effect. It allows countries in ruin to remain in ruin and dictators to remain in power. Intimidation within the context of only nominally free electoral politics represents a cheap and easy form of terrorism. Robert Mugabe knows this. Indeed his entire regime is based upon it.