l know that it increases water secretion, thus decreasing blood volume and thus lowering blood pressure, but can someone go into more detail?

Thanks!

p.s. How does a lower blood volume decrease BP?
and also are these used for CHF, because a decreased blood volume would decrease the load on the heart, thus allowing it to pump better?

Blood pressure is the pressure exerted against artery walls by the circulating blood volume. Reducing the volume within the same available space decreases the pressure against the walls of that space. Less pressure within vessels means lower blood pressure.
The heart has to work harder to pump against vascular resistance. With lower blood pressure, resistance to outflow from the heart is decreased, reducing the workload on heart muscle. In congestive heart failure, the heart acts as a weak pump, & blood circulating through the heart backs up from the left ventricle to the left atrium, & results in congestion within the pulmonary (lung) circulation. This allows fluid to leak from the vessels into the lung tissue. Reducing the volume of blood circulating also reduces pressure within pulmonary vessels, & reduces leakage (extravasation) into lung tissue.