Hail Mary is a last ditch effort when the countdown clock in an NFL game is near its end and the only real possibility to win the game is to lob the ball into the N-zone. This can sometimes be a throw of 50+ yards. It is tantamount to the idea in poker of all-chips-in strategy when a player has a diminished float and they have one last massive punt where they might, if lucky, win and remain in the game.
We have researched hundreds of traders using meta-trader manager and servers from large broking firms such as Pepperstone to analyse this behavioural error. I describe it as the Hail Mary trade. Even though it can be several trades and not just one, the trader typically has lost most of their account and has one or several last ditch effort to recover their account. They do this by placing much larger volume with the idea that their trading system is floored or their patience has run out and they are go to go all-in on a trade they believe is "the one".
From our research this never works, mostly due to margin ceiling where the broker will stop you out at 30% margin level.
The reality is the trader probably does not have a trading system or they have a system they do not believe in. By not understanding the probability of the outcome of their system they find themselves in a losing streak and decide to become a gambler rather than a trader.