In the field of electronics, a bootstrap circuit is one where part of the output of an amplifier stage is applied to the input, so as to alter the input impedance of the amplifier. When applied deliberately, the intention is usually to increase rather than decrease the impedance. Generally, any technique where part of the output of a system is used at startup is described as bootstrapping. Because the feedback is positive, such circuits can suffer from poor stability and noise performance compared to ones that don't bootstrap. For instance increase the input signal to 600mV and you'll see the stability bite the dust :D
In this particular circuit the bootstrapping is between the two emitters by means of a capacitor. Check the input impedance with and without the capacitor and you'll see what i mean.
