Sit on the floor with the legs straight forward and together and the trunk as upright as possible, with a dumbbell or kettlebell in one hand at the shoulder. Keeping your trunk braced and posture upright, press into a locked out overhead position, then return to the shoulder under control.   
 
Purpose
The Z-press position puts more demand on postural strength and control, and can help with shoulder and hips mobility. The single-arm variation allows more focus on that side, which can be helpful for shorting up asymmetry, and will reduce the demand on overhead mobility somewhat.
 
Programming
Generally sets of 8-15 are appropriate.