The data knife is always equipped on a Pilot; you do not need to actually switch to it to use it. When near an enemy Spectre, a prompt will come on screen "Hold X to Hack" and upon doing so, the Spectre and its squad will become your allies. The "Icepick" Tier 2 Pilot Kit unlocked at Pilot Level 43 will speed up the rate at which you hack a Spectre. There are a ridiculous amount of these in the final campaign mission, Chapter 9: Made Men.