The power formula is based on Equation 8.31 on page 336 of Rosner (2006).
$$
power=\Phi\left(-Z_{1-\alpha/2}+\frac{\delta\sqrt{n}}{\sigma_d \sqrt{2}}\right)
$$
where \(\sigma_d = \sigma_1^2+\sigma_2^2-2\rho\sigma_1\sigma_2\), \(\delta=|\mu_1 - \mu_2|\),
\(\mu_1\) is the mean change over time \(t\) in group 1,
\(\mu_2\) is the mean change over time \(t\) in group 2,
\(\sigma_1^2\) is the variance of baseline values within a treatment group,
\(\sigma_2^2\) is the variance of follow-up values within a treatment group,
\(\rho\) is the correlation coefficient between baseline and follow-up values within a treatment group,
and \(Z_u\) is the u-th percentile of the standard normal distribution.
We wish to test \(\mu_1 = \mu_2\).
When \(\sigma_1=\sigma_2=\sigma\), then formula reduces to
$$
power=\Phi\left(-Z_{1-\alpha/2} + \frac{|d|\sqrt{n}}{2\sqrt{1-\rho}}\right)
$$
where \(d=\delta/\sigma\).