At some point, there is a sustained increase in some type of spending for some reason. (One reason may simply be that, once inventories sink to their desired level, there stops being negative intended inventory investment, which goes up from negative to zero; but again there may be other reasons as well.) At this point there is negative unintended inventory investment as firms are caught by surprise by the external increase in demand and they fail to simultaneously raise their production. Now inventories are too low, again for two reasons, and we are back where we started in the cycle. The recession has bottomed out, sustained spending is once again high, target inventory levels are higher than actual inventory levels, and intended inventory investment is positive.