This certainly won't work against someone holding a gun although the chances of this happening in Japan are quite low.
In Japan, these things are all over schools, usually hanging on the wall or leaning in a corner. It is called a Sasumata, and used for pushing bad people. If a bad guy breaks in to school and starts running around with a knife or something then you are supposed to grab this thing off the wall and run at the bad guy with it. The ideal result is that the bad guy will become pinned to the wall or ground or get pushed out the door or something.
Source: The Low Down
Tags: Sasumata | Push