What is the "closed loop" with India?

The closed loop is the structural dependency: Bhutan exports hydroelectricity to India and earns rupees; Bhutan imports fuel from India and spends rupees back; the ngultrum is pegged 1:1 to the Indian rupee so the monetary stance is set by RBI; about a third of every export dollar flows back to Delhi as debt service on the Indian-financed dam loans. The book's Chapter Two unpacks each loop.