1) If selling has decreased substantially on a down move it’s a sign that sellers are exhausted. Supply has waned. The market will have a tendency to want to go up.
2) If buying has decreased substantially on up moves, demand has waned. The market will tend to want to go down.
So, it will take an increase of buying on up days to make the market go up, and an increase in selling on down days to make the market go down.