Consider the following quotation:
BioShock Infinite’s 1999 Mode will feature an especially demanding gaming experience, forcing you to examine your decisions while going through your adventure in Columbia. With every choice you make, there are irreversible implications, and if your choices guide you down a path not suited to your play style, you will suffer for it.
It’s not simply a matter of adjusting the difficulty sliders in the game – the team went much further than that. Resource planning? If you’re to survive this mode, proper planning will be crucial. Combat specializations? You’ll need to develop them efficiently and effectively throughout the story; any weapon will be useless to you unless you have that specialization. Combat? You will need to carefully target every shot, and your health will be set to an entirely different baseline. Game saves? Well, yes, there will be those, but according to Irrational Games Creative Director Ken Levine “there are game saves, and you’re gonna f***ing need them.”
“We want to give our oldest and most committed fans an option to go back to our roots,” said Levine. “In 1999 Mode, gamers face more of the permanent consequences of their gameplay decisions. In BioShock Infinite, gamers will have to sweat out the results of their actions. In addition, 1999 Mode will demand that players pick specializations, and focus on them.”
Pretty self explanatory. In short, choose your decisions wisely and play tactically.
As for how to unlock it, there are two ways.
Finish the game on any difficulty.
If you want an immediate challenge, use the konami code:
using the Konami code will allow them to unlock it at any point.
On PC (with keyboardmouse): Up, Up, Down, Down, Left, Right, Left, Right, (Cancel key), (Confirm key)
On Xbox: Up, Up, Down, Down, Left, Right, Left, Right, B, A
On PS3: Up, Up, Down, Down, Left, Right, Left, Right, O, X