This dialog provides for power analysis of a one-sample
t test or a pooled t test.  The effect size is the
difference between the null and the target mean (in the
one-sample test), or the mean difference of the pairs
(in the paired t test).

You may choose whether to solve for effect size or
sample size when you click on the "Power" slider.  The
current values are scaled upward or downward to make
the power come out right, while preserving (at least
approximately) the ratio of the two SDs or two ns.