The meaning of the 'Doorway Effect'

We have all experienced, at one point in our life, the so-called 'Doorway Effect': the fact of intending to do something, but immediately after having started, we forget what we were about to do. 

Our attention has the tendency to shift from one thing to another according to our ambitions, plans, strategies. It is at this exact point that the 'Doorway Effect' occurs.