Did you see Fig. 1 (on p.11) in the **broken link removed**? Read the accompanying text.
EDIT: Q5, along with R1, R2, R3, R4, and any external resistance you provide, in the National schematic forms current sources for Q1 and Q2. The Base-emitter voltages of Q1 and Q2 can be (un)balanced by adjusting these currents. That's what the balance circuit does. Positive feedback from the output to pin 5 also unbalances these currents, causing "offset voltage" in Q1 and Q2, which must be overcome by the input signal in order for switching to occur.