It’s when I have to be extra careful to be understood by the ladies. I have to speak slowly and use 1 or 2 syllable words. A syllable is a piece of a word. It’s kind of complex, and it’s not really too important. Don’t worry about it. Just know that it’s best to keep it simple.
The ladies are especially grateful when I speak slowly.
I’d add one thing to @Hawaii_Jake‘s excellent answer: Don’t use any numbers or calculations because we all know Math is hard and Math class is tough! .
When you have to explain something to someone who normally can’t follow in your regular speed/tone/language. If you take it too far you’re being condescending. If you don’t go far enough you’re being pompous. If you get it right nobody notices. The “mansplaining” part happens when you end up on the condescending side and the recipient is female and believes you’re being condescending because they’re female. Really it’s because they think you are a little dull, not because you are female.