What is the mechanism of "amiloride" (diuretic) that allow it to overcome the polyuria associated by taking lithium as a medication?

It prevents the entry of lithium to the nephron by blocking a sodium channel in the kidney tubing. I am no experience using this for that purpose. I defer to my kidney specialist colleagues.

By UPTODATE: amiloride inhibits the epithelial sodium channels in collecting tubule cells and therefore should minimize further accumulation of lithium (if lithium therapy is continued). However, amiloride is likely to be effective only when there is a mild to moderate concentrating defect that is potentially reversible.