This is a type of Astrology which is not of Indian origin but came to India via Persian influence. The principle is "Varsha Phala" or "yearly chart".
This is very useful because unlike the lagna chart, it helps in brief reading for one year period.
The chart is cast during the time when one enters a new year from his birth when the sun is in exactly same degrees of his birth chart. A chart is made during this time is known as Varsha Phala kundali.
Muntha is an important part of the Kundali. It changes one sign every year in a fixed manner. It is NOT planet but a progressive lagna (ascendant) which changes one house every year. So it is not actually muntha chart but year chart of which muntha is a part.