Do Not Swirl
https://blog.thenibble.com/2016/03/23/tip-of-the-day-use-snifters-for-food/for a list of things to do with your snifters other than destroy great spirits, like serving mashed potatoes in them, a fitting doom for these pretentious items.
It’s beyond painful to watch someone pour great cognac, or Germain-Robin single-barrel brandy, into a snifter, and swirl it.
The creator of any great aged spirit (rum and whiskey too) went to a lot of sophisticated trouble to create a balance of flavors, aromas, mouthfeel, and finish that play against one another on your palate. It takes a while for a truly great aged spirit to open up after you pour it: some alcohols are heavier/denser and need time to reveal themselves fully. When you swirl, especially in a wide snifter, you help evaporate the lighter alcohols prematurely and trash the spirit’s balance the blender went to a lot of trouble to create.
Anyone serious working with spirits uses a tulip glass (in photo), which focuses the aromatics a bit and dimishes evaporation so you get a more balanced sniff. Hold it maybe 2 inches below your nose – no swirling! – and inhale gently. A great aged spirit will take up to a half hour before the heavier components are fully open. A small white wine glass works fine.
The only thing WORSE is holding a lighter or a match underneath to warm the spirit. Simply cup the glass in your hand: that’s plenty.