The ELAS boys sit around the fire and listen to Hector "teach." Most of them don't agree with Hector at all and know that whatever Hector says, the opposite is likely true. Venizelists wonder how Hector can be so anti-British when the British are the only ones who have tried to help Greece. Mandras, however, is immediately sucked into Hector's discussion. Hector mentions that Lenin and Marx were intellectuals, but gave themselves over to raising up the workers--today, it's unnecessary to raise up workers; the workers must simply be taught to trust their intellectual leaders. He tells them why democratic processes will bring awful consequences, and why anyone who defects will be killed.
By including the note about the opposite of what Hector says being true, the novel links Hector to Mussolini and suggests that powerful leaders like them aren't to be trusted to tell the truth. Notice too that while Hector and communism purport to free the proletariat and put them in charge of their lives, Hector's insistence that workers must be taught to trust their leaders rather than raised up themselves implies that he's actually more interested in abusing communism to gain power than being truly communist.
Mandras pledges his support to Hector, hoping that one day he'll be able to read What Is To Be Done? himself. The narrator says that Mandras never will discover that the pamphlet is merely an irrational tirade against a rival communist newspaper, but he will learn the theory well enough to buy fully into the ideals of communism. After Hector's lesson, one of the Venizelists offers Mandras a cigarette and tells him that what Hector is basically saying is that they need to obey or they'll die.
The fact that the pamphlet itself isn't even what Hector says it is offers one more example of the ways in which powerful, enigmatic individuals can seize power by preying on uneducated and lonely individuals like Mandras. The Venizelist presumably knows how to think critically, which is why he sees it as a power grab.