Oscillator working

The usual explanation of a feedback oscillator goes like this. ""We have an Amplifier 'A(w)', a feedback circuit 'B(w)'. An initial input v=Ke(jwt) produses an output A(w)*B(w)*Ke(jwt) which in turn produces an output (AB)^2 * Ke(jwt) ...... => If the signal is to be sustained |AB|=1 and arg(A)+arg(B)=2*pi then each delayed echo or  cycle  of fluctuation will ?tack itself onto the tail? of the previous fluctuation with the same sinusoidal phase leading to oscillation."" I really don't get it . An Amplifier produces the output based on instantaneous value of input signal and there is no mechanism which stores an AC signal. Then how is the oscillation sustained if the initial disturbance is removed ? Please correct me if I am wrong.