On console you push the left anologue stick in you and sprint like in speed mode. On PC, you push left shift
It's not a mode you need to manually activate and lose other functionality in the process. It automatically activates when you sprint and also allows you to use other modes simultaneously, like sprinting in cloak or armour mode
One more thing, in armour mode you're slower (due to the suit has been hardened in this mode), so the sprint speed is slower, too.