Reflux divider is a pneumatic valve located between condenser and top of the column.
It helps in maintaining the desired reflux ratio in distillation column. It will be closed for some time and open for some time depending on the reflux ratio provided.
As of my knowledge, in manual operated distillation column, reflux ratio is operated in terms of time (considering vapor flow rate is constant) i.e., if you say reflux ratio is 5:1 then reflux divider will be closed for 5 sec (will not allow any condensate to pass through) and will open for 1 sec (allow the condensate to pass through). In other words, your condensate from condenser will return back to column for 5 sec and get routed out as distillate for 1 sec.
If you change the reflux ratio, the reflux divider action also change accordingly.