According to Wikipedia, a Hail Mary is a very long forward pass in American football, typically made in desperation, with only a small chance of success.
So, what’s a Hail Mary pass in chess?
Let me explain. Say your position is completely hopeless and you start thinking about resignation. First, do not resign! Then try to see if you can threaten a checkmate. While the chance of your opponent missing it might be very small, who knows what’s going to happen?
After all, they say “to err is human” for a reason. Threatening a checkmate is your Hail Mary pass! The following game shows that even the world’s best players use it:
Yes, as this game shows, in most cases a simple threat of a checkmate is not going to save you, but once in a while it can completely turn the tables. The following game is probably a record holder since it was published in nearly all the books on tactics that I read in my childhood.
Here is another famous example you have probably seen dozens of times. Black resigned in the following position thinking that a checkmate is unavoidable. What would you recommend Black do instead?
You can do a Hail Mary pass even in endgames, when there are not many pieces left on the board. In the following position, White resigned because he didn’t see how to stop Black’s pawns.
The following game was played here on Chess.com by one of my students. His opponent resigned due to heavy material losses…or so he thought. Can you find a Hail Mary pass here?
The conclusion is quite obvious: When in trouble, look for a threat of checkmate!