तद्वहति रथयुगप्रासङ्गम्

The affix यत् comes in the sense of 'what bears it' after the words 1. रथ 2. युग and 3. प्रासङ्ग being in the Accusative -2nd case in construction.
Thus रथं वहति = र꣡थ्यः 'what bears a car' a carriage-horse. Similarly यु꣡ग्यः 'a yoke-bearing ox', प्रासंग्यः 'being trained in a break'. प्रासङ्ग is a piece of wood placed on the neck of colts, at the time of breaking. This rule applies to compounds ending with ratha, as, परमरथ्य ॥ See 4.3.121, 4.3.123 also. That which carries will be called 'carrier' or वोढृ therefore रथस्य वोढृ will get the affix यत् by 4.2.120, read with 4.3.121. Its special mention here indicates that the affix will come, even after Dvigu compounds and will not be elided by 4.1.88. Thus द्वौ रथौ वहति = द्विरथ्यः ॥ The tadanta-vidhi applies to this word as we have shown under 4.3.121. The word युग्य has been already formed by the krit affix 3.1.121, that युग्य differs from the present, in accent when the word takes the negative particle, for that was by ल्यप् and this is by यत्, and therefore by 6.2.156, अयुग्य꣡म् will be finally acute when formed by यत् ॥ The word युगं must mean 'the portion of a car' for the application of this rule, and not a cycle of time &c. Therefore not here युगं वहति राजा कलिं द्वापरं वा ॥