the different between
i want to take a metaphor to explain.
下：you can imagine "drop,down,fall" if there is a drop rain,its light and soft;
if sth down or fall ,the speed fast（time fly）
then 一会儿=for a while.
一下as a drop；（more fast，happen in the twinkling of the eye）
一会as a line.（quick，fast）