Stack Flex gives you the possibility of 'stacking' two outlets on top of each other. By a series connection of the power supply's two outlets you'll end up with a higher voltage on the plug for your pedal. In the case of type 4 Stack Flex it's a green 5.5/2.5mm centre positive DC-plug. One of the power supply's two outlets used with the Stack Flex must be isolated.
With a type 4 plug, which is mainly used in 24 and 40V pedals from Electro Harmonix, typical applications of this Stack Flex are following:
using the last outlet of the Schizophrenic power supply set to 18V together with another outlet set to 9V you'll obtain 27V, which corresponds to the voltage a 24V pedal would get from an original 24V Electro Harmonix adapter which is the not regulated type, alternatively use two outlets set to 12V and then you get the nominal 24V
using the 18V outlet of the Power Bitch power supply together with one of the 9V outlets you'll obtain the needed voltage for another Electro Harmonix 24V pedal, so that your Power Bitch can power two pedals of that type
using the 24V and 18V outlets of the Power Bitch power supply you'll obtain the needed voltage for another Electro Harmonix 40V pedal, so that your Power Bitch can power two pedals of that type
using the Stack Flex with the last outlet of the Schizophrenic power supply set to 'adjustable mode' together with a 9V outlet it's possible to power a pedal with a voltage in the range 13-23V