Understanding bootstrapping in analog active filters
A simpler example would be a transistor voltage follower in which bootstrapping is used to increase the input impedance. Normally, the input impedance is significantly affected (reduced) by the bias resistors R1 &amp; R2.

By applying some of the output back to a series resistor R3 the input impedance is increased since the same voltage (or nearly) appears on either side of R3 so little current flows through it and the effect of the bias resistors is reduced.

With active filters, the situation is more complicated due to phase shift. Curd gives an explanation which could also be applied to bandpass filters but I'm not sure that there is any reasonable way of intuitively understanding how this works in the case of the S&amp;K LPF. Perhaps others know different!