Here is a sentence from an instruction passage of a game:
A sideways swipe will see the main character change direction or pull off a speedy mid-air dash
I am not sure if the word sideways means oblique or on one side here.
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"Sideways" means travelling in a relative direction of left-right or right-left.
In your example, a "sideways swipe" seems to mean a touch-screen gesture with your finger moving from one side of the screen to the other.
From the details you give, I'm not sure if you may be confusing the word "swipe" with the word "slash". For clarity, this is not talking about the symbol