The pedal uses op-amps (often TL072 or similar) to boost the input signal before it hits the tube. This "pushes" the tube into saturation more effectively.
A high-impedance stage that preserves your guitar's high-end before the gain stages. Blackstar Ht Dual Pedal Schematic
If you are looking at a circuit diagram, focus on these areas: The pedal uses op-amps (often TL072 or similar)
A complex arrangement of FET switches and logic gates that toggle between the "Clean/Crunch" channel and the high-gain "Lead" channel. Blackstar Ht Dual Pedal Schematic
Both halves of the tube are utilized. One half typically handles the initial gain stage, while the second provides additional clipping and drive.