Remember that your perception of how heavy the weight is, and how effective it is at stimulating your muscle to grow are not always aligned. What I mean is, you can't always tell by "feel" if the weight is too light.
As Carl mentioned, the effectiveness of any given load is dependent on the condition of the tissue at the time the load is applied. For example, if you have been using 100 pounds for curls for the last 6 months and then all of a sudden you start using 45 lbs because you are trying to follow an HST routine, you will be disappointed with the results.
Its not that it is impossible for 45 lbs to elicit a growth response, its just that in your conditioned state, your tissue is immune to the strain that 45 pounds produces. No damage, no trauma, no satellite cells, no nothing.
So, we take some time off before using lighter weight loads. If we decondition long enough for our tissue to re-sensitize, but not so long that we lose all of our gains, we will then be able to use much lighter weights during the initial part of our cycle and continue to see gains throughout.