XCold powder is one of Toko's standout products. It really is an essential part of any competitive skier's wax box. Here is information on when and how to use it:
XCold is not just a hardener. It has a special function. It makes the skis dramatically more slippery at slower speeds (in other words, they acelerate starting from a slower speed). The term that I like to use for this is that it improves the breakaway speed. For this reason, XCold is really important when conditions are slow such as very slow squeaky snow or slow freshly shot manmade snow.
You can apply it two ways. The most common way is to mix it with LF or HF Blue. The glide layer before this layer should have been a layer of Blue to harden up the base. Apply your Blue/XCold layer by dripping the blue on the ski (not too much, you need to leave room for the XCold). Then shake the XCold powder on the ski so you have about as much as the Blue. Then heat them in together, let cool, scrape, and brush. Some like to heat the Blue in first and then apply the XCold. I find that if this is done, the XCold stays on top and doesn't make it into the wax job.
Otherwise, in extremely slow cold snow, the XCold can be applied on top of the Blue as a separate layer. Simply shake the XCold on, heat it in, let cool, scrape, and brush. This is not as good as the Blue/XCold mix though except for in the coldest/slowest conditions.
If it is cold and fast, don't bother with the XCold, just go with the HF Blue followed by JetStream Blue.