SCIENCE:
Research has shown that 20 minutes a day of dual n-back training can significantly improve your fluid intelligence( IQ ) and working memory. The “dual n-back” is a memory sequence test in which people must remember a constantly updating sequence of visual and auditory stimuli.

HOW TO PLAY:
Dual N-Back task consists of a sequence of signals shown to you one by one. There are two types of signals in Dual N-Back (that's why Dual N-Back has "Dual" in its name):

audio signals — these are the alphabet letter sounds
visual signals — these are the squares shown on the training field

In the process of training you have to match signals shown to you now to signals shown earlier. This is what N-Back refers to.
For Dual 1-Back you have to match the current signal with signal shown on the previous step (one back); for Dual 2-Back - with the signal shown before previous (two back), etc.

CUSTOMIZATION:
This game is highly customizable. You can choose the N at the start of every game. Number of trials, duration of visual signal or time for response can be adjusted in the SETTINGS. You can also adjust the match rate (the frequency that any visual or auditory signal will match the one N steps back).
In addition to standard signals (position on the screen and letters) there are several other types available (images, colors, various sound effects, piano keys).

References:

https://releases.jhu.edu/2017/10/17/johns-hopkins-finds-training-exercise-that-boosts-brain-power/

https://www.pnas.org/content/early/2008/04/25/0801268105.abstract