The proposition consists of two parts: Part 1: This Tzimtzum caused.... This informs us that the effect of the Tzimtzum was to enable the Sefirot to be seen. Part 2: And this light... This provides further understanding of this effect.
Part 1: This Tzimtzum caused some of His light and radiance to become visible... For you surely already know and understand that what is beyond boundaries and limits cannot be seen. Initially the path of limitation and boundaries subsumed in Eyn Sof was also invisible, inasmuch as it was subject to His all-encompassing limitlessness, which turns all limits back beyond limits. But when the Tzimtzum removed the aspect of limitlessness from this pathway, the pathway remained there and now became visible. This is what caused...some of His light… to become visible... This is the light of which we spoke earlier, which Eyn Sof wanted to be accessible so that the attributes of the supreme law would appear by way of radiations of light – the Sefirot.