What does 'strong or weak day-master' mean in Four-Pillar astrology?

'Strong' or 'weak' body in BaZi refers to the day-master’s relative strength within the whole chart. This assessment determines strategy: whether to support the day-master (if weak) or to restrain/release it (if strong).

Three main criteria to judge strength:
1. In-season (De Ling): Is the day-master born in a month that favors its element? The month’s influence is the most important factor.
2. Rootedness (De Di): Does the day-master have roots in the branches (supporting branches containing the same element)?
3. Backing (De Shi): Are there helpful stems (resources or companion stems) that strengthen the day-master?

Types and implications:
• Strong day-master: When the day-master is in season and has roots/backing. Characteristics: energetic, resilient, able to shoulder responsibility. Strategy: 'use' stars that consume or control it (wealth/official) to balance.
• Weak day-master: Lacking season, roots, or backing. Characteristics: sensitive, dependent, less able to take initiative. Strategy: promote/support the day-master with resource stars (printing) or peers.
• Extreme patterns: Very strong or very weak charts may require special rules (e.g., 'conforming' to the dominant element or following 'from' patterns).

The goal of analysis is balance: identify the chart’s imbalance and choose a helpful element to restore harmony—this is the basis of 'fuy-yi' (supporting and restraining) and selecting the yongshen.