A seriously ferocious dippin' sauce.
Hot mustard is traditionally used in Chinese cuisine, often served along with sweet and sour sauce with egg rolls or wontons. The "hotness" of hot mustard is very peculiar; it can be described as "pungent" and "hot," but it's not the same kind of "hot" one would expect from eating chili peppers. I would wager that it's closer to a horseradish/wasabi "hot" than anything else, blissfully clearing one's sinuses while rapidly disappearing from the palate with nary a trace of its presence. Whereas the heat of chili peppers comes from capsaicin, the heat of hot mustard is a result of various sulfur-containing compounds (particularily myrosin and sinigrin) that are broken down by a companion enzyme when the mustard is ground and mixed with water.
Hot mustard can be made easily -- simply combine equal parts cold water and dried, ground, mustard seed. Mix well, let it sit for ten minutes or so, and the enzymes will have broken down the sulfur-compounds sufficiently. The quality and pungency go downhill from this point, so don't make too much. It's best to use it immediately; if you must store it, keep it refrigerated and airtight. Add a little rice vinegar or wine as a preservative if you must, but only after having enjoyed it in its purest state. If you find it's a little too hot for your liking, you can maintain the same consistency by adding a little vegetable oil to the mixture, or if it's too thick, use a little more water.
So why is it that mere mustard powder and water transform into such a potent, pungent condiment, whereas the mustard you buy at the store is so wimpy? Take a look at the ingredients. Hot mustard is 50/50 water and mustard by volume; this generic bottle of yellow mustard I have at my desk reads: distilled vinegar, water, mustard seed, salt, turmeric, paprika, spices ("spices" is likely MSG in sheep's clothing). So, whereas hot mustard is little more than pungent mustard powder and water, yellow mustard has more vinegar and water than anything else by far. The vinegar seems to take the edge off of the heat substantially, but the mere fact that it's so diluted (and is kept at room temperature on your grocer's shelf for weeks/months/etc) explains it all.
In my opinion, this stuff rules, but I may be biased due to my affinity for pain-inducing foods, as well as my persistent springtime nasal allergies (mustard seems to instantaneously clear my sinuses and nasal passages, and I really mean instantaneously). In my opinion, mustard isn't really a painful food, as as I mentioned above, the taste seems to dissipate rather quickly. It's great to dip egg rolls or crab cheese wontons (I know, they aren't real Chinese food) in both mustard and sweet and sour sauce to slightly diminish the effects. Whereas most people in my experience tend to use hot mustard exclusively with appetizers, I love nothing more than to dip sesame chicken, sweet and sour chicken, beef with broccoli, etc, etc, into hot mustard and sweet and sour. If you order Chinese take-out and get a bunch of left-over packets, keep them at work and next time you pick up some greasy chicken nuggets from a fast food joint, be sure to get sweet and sour sauce (Wendy's has it, at least) and enjoy some poor-man's wannabe sweet and sour chicken with hot mustard. Yummixor.