In Bazi destiny analysis, chart construction and Ten God identification are merely preparatory work. The true core technique lies in judging the strength of the Day Master and selecting the "Useful God" (Yong Shen) accordingly. The Useful God is the Five Element or Ten God in the Bazi chart that is most beneficial to the Day Master and best able to balance the chart. Finding the correct Useful God is like finding the key to unlock the door of destiny; if the Useful God is wrong, the entire analysis is wrong. Therefore, destiny analysts throughout the ages have regarded Useful God theory as the soul of the Ziping Method.

Judging the Day Master's strength requires comprehensive consideration of four factors: being in season (De Ling), having roots (De Di), having momentum (De Shi), and being nurtured (De Sheng).

Being in season means whether the Day Master is born in the season when its element is dominant. For example, Wood is in season in spring, Fire in summer, Metal in autumn, Water in winter, and Earth in the last month of each season (Chen, Xu, Chou, Wei). Being in season is like a plant growing in the appropriate season—it has superior innate conditions. If the Day Master is out of season, it is like sowing seeds in the depths of winter—innately deficient.

Having roots means whether the Day Master has roots in the Earthly Branches of the chart. "Roots" refer to Branches that share the same Five Element as the Day Master, or Branches whose Hidden Stems contain the same Five Element as the Day Master. For example, if the Day Master is Jia Wood, encountering Yin or Mao in the Branches, or finding Jia Wood hidden within Yin or Yi Wood hidden within Mao, constitutes having roots. The more roots, the more stable the Day Master. If the Day Master has no roots in the Branches, it is like duckweed without roots—difficult to stand independently.

Having momentum means whether the elements that support the Day Master hold the advantage in the chart. Those that support the Day Master include: Siblings and Rob Wealth (same Five Element as the Day Master), and Resource stars (those that generate the Day Master). If the chart contains many and strong Sibling and Resource stars, the Day Master has momentum. Conversely, if the elements that restrict, drain, and consume the Day Master—Officer, Output, and Wealth—hold the majority, the Day Master loses momentum.

Being nurtured means whether the Day Master directly receives the nourishment of Resource stars. The Resource star is the Ten God that generates and supports the Day Master, like a mother nursing her child. If the chart contains a Resource star close to the Day Master and in a strong position, the Day Master is nurtured.

By synthesizing these four points, one can determine the strength state of the Day Master. Generally, if the Day Master possesses three or more of these four conditions, it is considered strong; if it possesses only one or none, it is considered weak; if it falls between these two extremes, it is considered balanced.

After determining the Day Master's strength, the next step is selecting the Useful God. The basic principle of Useful God selection is: for the strong, use restriction, drainage, and consumption; for the weak, use generation and support.

When the Day Master is strong, it needs restriction, drainage, and consumption to achieve balance. Those that restrict the Day Master are the Officer stars; those that drain the Day Master are the Output stars; those that consume the Day Master are the Wealth stars. Which one to select as the Useful God depends on which element is most lacking in the chart. If the Officer star is strong, it can be selected as the Useful God; if the Officer is too weak or being controlled, the Output can be selected; if the Output is also weak, the Wealth can be selected. For example, if the Day Master Jia Wood is strong, and Fire, Earth, and Metal all exist in moderate strength, Geng Metal (Seven Killings) can be selected as the Useful God, using hardness to control hardness, leading to achievements in military or management positions.

When the Day Master is weak, it needs generation and support for nourishment. Those that generate and support the Day Master are the Resource stars; those of the same category are the Sibling stars. If the Day Master is weak but the Resource star is strong, the Resource can be selected as the Useful God, using the Resource to transform the Officer and generate the body; if the Resource is weak but the Siblings are strong, the Siblings can be selected as the Useful God, using them to support the body and resist the Officer. For example, if the Day Master Geng Metal is weak, born in the summer when Fire is prosperous, and if there is an Earth Resource star in the chart, Earth can be selected as the Useful God, using Earth to generate Metal and resolve the danger of Fire restriction.

After selecting the Useful God, one must also determine the "Favorable God" (Xi Shen) and "Unfavorable God" (Ji Shen). The Favorable God is the element that assists the Useful God; the Unfavorable God is the element that restricts or harms the Useful God. For example, if the Useful God is Wood, then Water generates Wood, making Water the Favorable God; Metal restricts Wood, making Metal the Unfavorable God. When analyzing Major Luck and Annual Luck, encountering the Favorable God brings auspiciousness, while encountering the Unfavorable God brings misfortune.

Day Master strength and Useful God selection are the most skill-testing aspects of the Ziping Method. They require the destiny analyst to not only master the Five Element relationships and Ten God interactions but also to possess comprehensive analytical ability and rich practical experience. If the Useful God is accurately identified, the judgment of destiny can hit the mark; if the Useful God is misidentified, the analysis may lead to the opposite conclusion. This is also why the Ziping Method is called an "Art" (Shu) rather than a "Science" (Xue)—it is not merely the accumulation of knowledge but the refinement of skill.

在八字命理学中，排盘和定十神只是准备工作，真正的核心技术在于判断日主的强弱，并据此选取"用神"。用神，是八字中对日主最有益、最能平衡命局的那种五行或十神。找准用神，就找到了打开命运之门的钥匙；用神一错，满盘皆错。因此，历代命理学家都将用神理论视为子平术的灵魂。

判断日主强弱，需要综合考虑四个方面的因素：得令、得地、得势、得生。

得令，是指日主是否生于当旺的季节。例如，木生于春、火生于夏、金生于秋、水生于冬、土生于四季末（辰、戌、丑、未月），即为得令。得令者，如同植物生长在适宜的季节，先天条件优越。若日主失令，则如同寒冬播种，先天不足。

得地，是指日主在四柱地支中是否有根。根，即与日主五行相同的地支，或地支藏干中有与日主相同的五行。例如，日主甲木，地支见寅、卯，或寅中藏甲木、卯中藏乙木，皆为得地。根气越多，日主越稳固。如果日主在地支中无根，如同浮萍无根，难以自立。

得势，是指八字中帮助日主的五行是否占据优势。帮助日主的包括：与日主同五行的比肩、劫财，生助日主的印星。如果八字中比劫和印星数量多而有力，则日主得势。反之，如果克泄耗日主的官杀、食伤、财星占据多数，则日主失势。

得生，是指日主是否直接受到印星的生扶。印星是生助日主的十神，如同母亲哺育子女。如果八字中有印星紧贴日主，且印星有力，则日主得生。

综合以上四点，即可判断日主的强弱状态。一般而言，得令、得地、得势、得生中占三项以上者，日主偏强；仅占一项或一项都没有者，日主偏弱；介于两者之间者，日主中和。

确定日主强弱之后，下一步就是选取用神。用神选取的基本原则是：强者宜克泄耗，弱者宜生扶。

日主强旺时，需要克泄耗来平衡。克日主者为官杀，泄日主者为食伤，耗日主者为财星。具体取何者为用，要看八字中哪种五行最为缺乏。如果八字中官杀有力，可取官杀为用；如果官杀太弱或被制，可取食伤为用；如果食伤也弱，可取财星为用。例如，日主甲木强旺，八字中火土金都有但力量适中，可取庚金七杀为用，以刚克刚，成就武职或管理才能。

日主衰弱时，需要生扶来补益。生扶日主者为印星，与日主同类者为比劫。如果日主弱而印星有力，可取印星为用，以印化杀、以印生身；如果印星无力而比劫有力，可取比劫为用，以帮身抗杀。例如，日主庚金衰弱，生于夏季火旺之时，若八字中有土印星，可取土为用，以土生金，化解火克之危。

用神选取之后，还需要确定"喜神"和"忌神"。喜神是辅助用神的五行，忌神是克制或伤害用神的五行。例如，用神为木，则水生木，水即为喜神；金克木，金即为忌神。在分析大运和流年时，遇到喜神则吉，遇到忌神则凶。

日主强弱与用神取用，是子平术中最考验功力的环节。它要求命理师不仅熟练掌握五行生克、十神关系，还要具备综合分析能力和丰富的实践经验。用神取准了，对命运的判断就能切中要害；用神取偏了，就可能南辕北辙。这也是子平术被称为"术"而非"学"的原因——它不仅是知识的积累，更是技艺的磨练。