When you press the button both transistors are enabled, but the coil keeps off because it is supplied by 2,5 Volts.

By releasing the button, the capacitor keeps on the Q2 transistor for a bit; the relay turns on because the coil takes about 5 Volts. At this point the Q2 transistor keeps on because the relay contact is closed between 14 and 11.

When the button is pressed again the Q1 collector turns down the base of the other one. The coil is supplied by 2,5 Volts and keeps on due to the hysteresis. By releasing the button the relay backs to the start condition.