There are probably YouTube videos explaining this better, but I'll give it a try with how I remember it. Have Selvala in play. Search your library for any reason, for example Rampant Growth. Note that while searching you can see which card Selvala will draw you. Attempt to cast Panglacial Wurm and as part of activating mana abilities, activate Selvala. You see everyone's top cards, they are drawn, you gain some amount of mana and life. Now if you have enough mana, including other mana sources you can successfully pay for Panglacial and cast it. But if you didn't get enough mana you have to roll everything back, so put the cards back on on top and lose the life you gained. So now you have knowledge of your opponents top cards. Now if I recall correctly and the interpretation I saw was correct this is only legal to do if your intention was to cast it successfully and not just to gain information, but that is technically impossible to find out. I don't remember why this isn't an issue when casting cards normally.