Urad dal used whole in thoran/rasam - how to correctly prepare? South indian recipes often seem to call for adding dry/rinsed raw urad dal (or even bigger pulses) straight into the oil with the spices at the beginning, or adding them into a tadka that is added later .. as expected, naively following that advice often leaves some or all of them very hard/crunchy/sometimes inedible in the finished dish. Is there a soaking/parboiling method that should be used and that recipe writers tend to assume you know to employ, and/or are there rules of thumb how the dish being cooked must/must not be treated after adding the raw dal or tadka to avoid such errors?
I believe raw urad dal is meant to be split urad dal as mentioned by @Athanasius .
Any dal you use make sure it is split and not whole otherwise i will not temper properly.
Whole dal becomes too hard or burnt in oil.
For me the dal becomes crunchy and it is supposed to be a bit hard(not teeth breaking hard but like bits of corn seeds in pop corn).
It would never become completely soft as you are tempering it. Rather than soaking in water i would suggest roasting them in oven or low gas heat before you use it to temper if your dal stays to hard for you.
Like Daliya (Roasted chana dal) that we use in Idli chatney, gives nice crunch to your chutney. I hope this helps.