Think of the voltage on the base of Q1 as effectively being across the 330 ohm emitter resistor thereby modulating the Q1-Q2 current. The same thing is happening with Q2. If Q2 is "more off" the the voltage at the collector of Q1 will be high, and vice versa. Note that there is no dot between the diodes. The diodes just sorta parallel the two Vbe diodes on the output devices. The 1M feedback resistors kind of clean things up.