If you are new to a dungeon, a simple "Hi, I'm new to the dungeon." is usually enough to let the others know they should take it easy or explain a key mechanic.

As for explaining it myself, if it's a fight that I know has a special dynamic, I'll ask "Does everyone know how this goes?". I figure it's easier to take a few seconds to explain it NOW rather than spend minutes dying or wasting mana LATER. It's an expediency thing.