The first thing to keep in mind is that Arendt was one of the earliest theorists dealing with this issue. "The Origins of Totalitarianism" was published in 1951, we means she probably started writing it just at the end of WWII. I'm not certain her thoughts are fully developed on the issue. Certainly they are only loosely integrated into her later wok on democracy and the vita activa. Just so it's said...
But that point aside, Arendt defines a totalitarian regime as something that goes beyond mere authoritarianism or autocratic behavior. Where an autocratic or authoritarian regime seeks to gain power and control — to impose its will on citizens by force — a totalitarian regime seeks to dominate every aspect of its citizens' lives. In effect, it tries to undercut the capacity for effective opposition at a cognitive level. So when Arendt refers to the "perpetual-motion mania of totalitarian movements," she is pointing to the fact that totalitarians pour out endless streams of propaganda, disinformation, lies, recriminations, self-aggrandizements, conspiratorial ideation: anything, in fact, that can disrupt, suppress, or overwhelm the capacity for calm, reasoned deliberation. Totalitarianism is a continual "Swift Boat" (or if you prefer, blitzkrieg) strategy in which constant forward motion is used in place of strategy, where political attacks or moments of propaganda are committed one after another after another, and dropped as soon as the opponent starts to catch on. The aim is to keep things moving so quickly that one can wound one's political opponents without ever giving those opponents the time or targets to martial a counter-attack, and to keep the populace in an emotionally heightened and cognitively diminished state, so that they react only to the apparent 'success' of the totalitarian strategy, not to its (lack of) substance or content.
If you've seen the old Star Trek episode where a Klingon says to his commander "A running man can slit a thousand throats in a night," well... that attitude lies at the heart of totalitarianism.
Now, this is where that line about 'extraordinary adaptability and absence of continuity' comes into play. For the totalitarian mindset, philosophical universals — things like truths, ideals, moral standards, ideologies, etc — are things to be attacked, not things to hold. They are considered weaknesses. The totalitarian is constantly looking for ways to aggrandize his group and denigrate others, and it does not matter to him if his words, actions, or attitudes are completely inconsistent from one moment to the next. It's all a game to maximize his sense of prestige, and prestige comes from making himself look smart and strong while making his opponent look foolish and weak. The names of Hitler and Stalin were largely dropped after their deaths because the names were vilified, and invoking them was a position of weakness. Now they are coming back in vogue because the political climate has changed: invoking Hitler's or Stalin's name is likely to produce an emotional reaction that will make opponents look weak. None of the modern neo-Nazis care one wit about Hitler; Hitler is merely a red flag they can wave at the bull, to disrupt reasoned though and generate emotional distress.
In a way, it's like what Anton LaVey said when he formed the Church of Satan: he didn't believe in Satan or the devil or God; he just thought that basing his organization around Satan would generate conflict with Christians.
The point Arendt is trying to make is that the totalitarians who follow a particular leader will have loyalty to that leader only so long as he is alive and in charge. The moment he dies, he becomes an inconvenience to their efforts to dominate — maintaining loyalty after death would be perceived as a sign of weakness that would be attacked — so they are just as happy to abandon him. And having abandoned him, they may very well pick his name and image up again if it becomes an effective tool. Totalitarianism is, in this sense, a perfection of nihilism, in which nothing matters except one's own overweening quest for dominance.