Generally I like it jumping straight in, but "it depends". (If anyone says "test it", generally I'd prefer you to be spending that time making the game better.")
I would say "test it" because more users usually means better chances of getting feedback on how to make the game better.
Other than that, I think the mental cost of having a paragraph explaining what the game is and having to click a "PLAY NOW" button is pretty low. Just make sure you explain it well.
