To understand the inverted (or more properly, "averse") pentagram, we must first examine how the pentagram itself is formulated.
Diving right in, most modern magical traditions assign elemental associations to the five points of the pentagram: Earth, Air, Fire and Water. Most often these are in the lower left, upper right, lower right and upper left points respectively. The Tarot trump "The Universe" (or "The World") is a key to this: it shows a woman in the center of the card, with the four kerubic angels surrounding her in the quarters appropriate to their elements. If
elemental spirit were shown on the trump, it would be above the woman. If spirit averse were shown, it would be below.
The top point is reserved for spirit, and the pentagram as a whole symbolises spirit rising out of the physical elements. As such, it also symbolises the release of energy from matter in such processes as fire, prayer and physical death.
The averse pentagram is formulated in precisely the same manner, except that the point assigned to spirit is below the other four (which retain their original positions). As such the averse pentagram symbolises spirit energy embedding itself into matter, representing such processes as Initiation, Consecration and physical birth.