When Ib flows all the diode act like a short circuit (or low impedance) when there is no current all the diodes are off and the current path from input to output is via the reverse junction of the diodes. The difficulty with the circuit is keeping voltage drops associated with the switching pulses out of the storage capacitor, so a balanced (about earth) current drive is what you want. Also when the switching pulses are there, you don't want your input signal going into the switching current source so this must be high impedance.
Frank