The concept of save scumming is simple: exit NetHack via the Shift-S save command, then copy the saved game from NetHack's playground to a different directory. Run NetHack again, and continue playing until you die. At this point, restore the saved files, copying them back from the temporary directory to the playground, and restart the game. Assuming the correct files were copied, the game continues as it was. This process is cheating, especially because the legitimate way to sidestep death is explore mode.
Save scumming can do more than allow a player to avoid death. Indeed, just about any random element (such as which prize lies at the end of Sokoban, or how much damage a monster inflicts this turn) can be sidestepped with save scumming.
On a few systems, it might be necessary to guard against game or system crashes, because some copies of NetHack lack a working recover program. An other legitimate use, would be to use save scumming in place of explore mode, while learning how to play.
A variant of save scumming would be to transfer the save file to an other computer. When doing this, beware, that save files aren't compatible between versions or platforms, not even between 32 and 64 bit builds. You must use the same executable, for this to work with no issues.
Specifically, if you are transfering from a 32bit linux to it's 64bit version. It's easier to just install the 32bit version of nethack in the 64bit system. In ubuntu that can easily be done with
sudo apt-get install nethack:i386
For more different platform, you'll need compatibility layers if available, or even full blown emulation.
NetHack is designed so that death is permanent (except for the amulet of life saving, of course). This adds difficulty to the game, but also adds depth. When making a mistake might cost a week of gameplay, one thinks longer about each action, and investigates more fully the possible repercussions. It also adds excitement: taking risks with a carefully cultivated character becomes more exhilarating when the stakes are higher.
While savescumming may allow a new player to go farther in the dungeon than he or she has before, it will ultimately create a weaker player than if he or she had toughed it out. By saving and restoring games, the savescummer avoids the learning of the intricacies of NetHack, and will never learn the art of extracting him or herself from difficult positions. Death is part of the game, and a player who savescums to avoid death is not truly playing NetHack. NetHack is like playing several games of chess at the same time. Save scumming is like setting up the board so that you will win without having to think about anything at all. The ONLY reason you should back up your games is to avoid losing whole weeks of characters due to system crashes.
There are some cases where saving a game at a difficult choke point and then playing through multiple ways, trying to find a solution, can be more enlightening than dying and starting over. If you do this, you should probably not continue and finish the game for the reasons explained in the rest of this article. Using the save-preserving feature of explore mode may be a preferable means of accomplishing this. Alternatively, you can re-create the situation in a separate wizard mode game, do the testing which solution is best, and then go back to your original game. This sidesteps the entire "cheating" issue.
There are many roleplaying games that allow you to avoid death by saving and restoring your game. By savescumming, a player is missing out on the true fun of the game, at which point NetHack is far less superior to any other RPG available.
But, on the other hand...
This wiki is a nest of spoilers designed to smooth over the same bumps and pitfalls that a player may seek to avoid via save scumming. So who are we to judge, eh?
The easiest way to savescum is to start a brand new game. Once it is started, examine the directories in which NetHack is storing data and find one with a name similar to "save". Now save and exit the game. Examine the directory again, and see what files have appeared. Make a list of the files and the filename syntax (usually the player name with a number). Now restart the game and let the character die. Once again, examine the directory and see which files disappeared. After doing this you should have a good idea of what files hold the game's information.
One can write scripts to speed this process up.
NetHack provides no way to save a running game, as the game file is only created when you save and exit.
On multiuser systems
However, this problem can be circumvented by running as root and using cp -p which will preserve the permissions to the file.