What it is
Khavedamsha divides each sign into forty equal parts of 45′, three-quarters of a degree. It is one of the higher vargas, used only with a reliable birth time.
How it is calculated
Rule: in an odd sign the first part is Aries, in an even sign Libra; then onward in order, so the zodiac turns more than three times across the 40 parts.
Example: a planet is at Aries 17°. 17 ÷ 0.75 ≈ 22.67, so the twenty-third part. Aries is odd, so the twenty-third sign from Aries: Aquarius (a full round of twelve, then the eleventh). In D40 the planet is in Aquarius. At Taurus 17° it would be the twenty-third from Libra, that is Leo.
What tradition looks at in it
Parāśara links the khavedamsha to the general good and difficult experiences of life. Many astrologers of later tradition read in it the legacy of the mother's side, the values and support that come from the maternal family. The Moon and the lord of the 4th house are looked at. It is a fine, supporting view of general well-being.
How much the birth time matters
On average the ascendant moves about 1 degree every 4 minutes; some signs rise faster, some slower. One part of D40 is 45′, so a difference of roughly 3 minutes in birth time can change the D40 ascendant. Even the Moon crosses one part in about an hour and a quarter. In your chart, ePandit shows next to every varga how many minutes of birth-time change would shift its ascendant. Most people's birth time is not this precise, so hold D40 lightly.
How to read it calmly
Read D40 as a second look, not a verdict. Tradition first studies the 4th house and its lord in the main chart (D1), then the running dasha. D40 only helps check whether the same theme shows up here as well. A planet's sign in this varga decides nothing on its own. A family's legacy holds much that a chart does not show: stories, habits, love. Treat this varga as only a small part of it.