According to University of Oxford:
In this new experimental study the researchers compared the effectiveness of Tetris at reducing flashbacks with Pub Quiz Machine 2008, a word-based quiz game. They found that whilst playing Tetris after viewing traumatic images reduced flashbacks by contrast playing Pub Quiz increased the frequency of flashbacks.
In the first experiment, after waiting for 30 minutes, 20 volunteers played Tetris for 10 minutes, 20 played Pub Quiz, in which they had to select one of four on-screen answers, for 10 minutes and 20 did nothing. Those who had played Tetris experienced significantly fewer flashbacks of the film than those who did nothing, whilst those who played Pub Quiz experienced significantly more flashbacks.
‘Our latest findings suggest Tetris is still effective as long as it is played within a four-hour window after viewing a stressful film,’ said Dr Emily Holmes of Oxford University’s Department of Psychiatry, who led the work. ‘Whilst playing Tetris can reduce flashback-type memories without wiping out the ability to make sense of the event, we have shown that not all computer games have this beneficial effect – some may even have a detrimental effect on how people deal with traumatic memories.’
Apparently, the paths that are used to set the responses to the trauma are engaged by Tetris, and thus prevented from solidifying the trauma? Something like that.