What is the ingredient that gives a sichuan (szechuan) hot pot the mouth numbing experience?
Depending where you live it can be hard to get hold of useful Szechuan pepper. What you want is just the outer shell of fruit, not the contents

Many governments force importers from China to use high heat to sterilise these on import, and most of the active ingredients seems to get whacked during this process

Find the most "Chinese" shop you can, and buy packets that look like they where sealed in China (No local language print or contact detail, only Chinese characters). These hopefully haven't been heat treated and will still be good. Keep it airtight until needed

Sometimes it's labelled "Red Prickly Ash" or "Prickly Ash", or look for this on the label 花椒

To use, lightly dry roast the whole shells and grind into a powder. Then measure the fresh ground powder to match the recipe. It is only a subtle effect compared to Kava or Kawakawa and I think it needs lots of chilli too taste great