As far as Semdzins go, I would like to paraphrase Namkhai Norbu Rinpoche in his book Dzogchen teachings, from pages 93-99.
Basically he explains and says that Trechod is impossible without having prior knowledge of our condition.
Let me say that again: Impossible. That means no real presence. Now sure we can awaken knowledge through our practices of presence and so forth, but there is doubt if there is no knowledge. Then our practice isn't coordination of our body, speech, and mind with this knowledge but too much effort, usually.
In these few pages ChNNR doesn't actually mention Semdzins. Nevertheless, the express purpose of Semdzins is to discover this knowledge.
Now my take is that whether we have knowledge or not, if it is not very stable, then we can more easily affirm or reaffirm our knowledge through Semdzins. Or if we don't have knowledge, we can gain it through Semdzins.
This only helps our practice of presence.
Knowledge is key. It's just like anything else. Save here it is not so much intellectual. Though of course we need intellectual knowledge of the things taught to us as well.
"When we are following Dharma, any kind of Dharma - Sutra, Tantra - most important is that first of all we know our condition, not Dharma". - The maestro ChNN