In this situation ions inside insects tend to move down their concentration gradients to their relatively dilute, or hypoosmotic environment. There is a general requirement that this loss of ions be offset. More immediate to our story, however, is the observation that if the potassium ions in the urine were not somehow recovered, an insect would soon be unable to continue excreting excess water. Because the movement of excess body water follows passively from the active movement of potassium ions. The physiological solution, as you already well know, is that the complete excretory system of insects includes the rectum.
Secretion is by way of Malpighian tubules and reabsorption by way of rectum.
In both of these exemplars the rectal fluids are very dilute relative to the Malpighian tubules and hemolymph. The general idea is potassium is actively pumped into the lumen of Malpighian tubules. Water accompanies this movement, along with some amounts of other salts, and there is a concomitant bulk flow into hindgut and thence to rectum. Before the water is voided, salts are again actively pumped back into the extracellular compartment by specialized regions of rectum. This second pumping of ions produces a very dilute urine which can be voided while ions are kept within bodies.