Green’s Dictionary of Slang

x n.1

also letter x
[the crossed position of the arm]

a method of arrest whereby a policeman grasps the villain’s collar and holds their arm in such a way that the more they struggle, the more likely it is for the arm to be broken.

[UK]Hotten Sl. Dict. 272: X, or LETTER X a method of arrest used by policemen with desperate ruffians, ? by getting a firm grasp on the collar, and drawing the captive’s hand over the holiday arm, and pressing the fingers down in a peculiar way ? the captured person’s arm in this way can be more easily broken than extricated.
[Aus]Sydney Sl. Dict. (2 edn) [as 1864].
[Aus]Crowe Aus. Sl. Dict. 97: X, as ‘I’ll make an x of you,’i.e., twist you in such a way as to make it impossible for you to walk.