Accelerometer Interrupt Meaning - Wakeup Function
The INT_GEN_CFG_XL 6-Direction detection interrupt apparently is aimed at detecting change of direction of travel, rather than an axis-combined acceleration level.

See the LSM303 discussion at Polulu.com.  That page includes a link to App Note CD00290365.pdf for the LIS3DH.  Here is what that app note says, on page 27:


  There are two possible configurations for the 6D direction function:  
  
  
    • 6D movement recognition: In this configuration the interrupt is generated when the device moves from a direction (known or unknown) to a different known direction. ...
    •  6D position recognition: In this configuration the interrupt is generated when the device is stable in a known direction. ...
  


With the LSM9DS1 that you are using, you can leave OR enabled, set separate X,Y,Z threshholds in INT_GEN_THS_X_XL, INT_GEN_THS_Y_XL, INT_GEN_THS_Z_XL, and set the “Enable interrupt generation on accelerometer's X axis high event” (sim. for Y and Z) bits in the INT_GEN_CFG_XL register.