How will the dew differentiate between Jewish and non-Jewish dust? Here's his example: Imagine someone spread iron filings all over a dark sandy floor. There's no distinguishable difference between the different kinds of dust.. but pull out a magnet and instantly discerning which dust particles are iron filings becomes trivial.
Similarly, when the final resurrection takes place, only then will the different nature of the Jewish People become apparent. This, R' Hayyim explains, is hinted to in the letter vav of the word פדות (pedut) which is not written in the text, but is nonetheless pronounced.