this depends if it is chipped on not.

As a general rule, if it is chipped, the knock system is disabled so earthing
the wire will have no effect unless the chip maker uses it for other purposes.

On the standard chip, earthing this wire reduces the knock sensitivity.
I.E. The sensor becomes less effective at retarding ignition.
This is used for two reasons........
1) Low octane fuels (NOT RECOMENDED)
2) An old engine generates more mechanical noise and can falsely trigger the
sensor and so reduce engine performance as the ignition retards.