Drig Bala is the aspectual strength of planets, i.e. strength received from other planets by their aspects.
Most Balas are standalone properties of a planet. But Drig Bala has to do with strength coming from other planets. This is important for prediction because periods of a planet with high Drig Bala will produce helping effects. Aspecting planets will help during their periods or transits.
The classical view of Graha Drishti is refined in chapter 26 of Hora Shastra: Sputa Drishti gives exact values for each angle between pairs of planets, see the Drishtis (Aspects) page.
Drig Bala has a slightly different approach. Aspects are sorted in two groups: benefic and malefic aspects.
Benefics are Jupiter and Venus. Malefics are Sun, Mars and Saturn. Moon is a benefic if birth takes place in the bright half of the month. Mercury is malefic if conjunct with a malefic (including nodes).
Drig Bala needs a rectification of Sputa Drishti. The values for rectification are
The Drig Bala of a planet is the sum of all rectified aspectual values.