Hayanaratna 5.183

This is the meaning. At birth, Jupiter is lord of the 5th house. He looks at the 5th house, or stands in it. So he can give a son. Then comes a year when Jupiter's dasha (planet period) runs. In that year the year-lagna lord and the year lord form israpha (planets moving apart after an aspect) with Jupiter. Then gain of a son is not to be told for that year. In the opposite case, the result is to be told. That is, when there is no israpha because the deepta degrees (the planet's orb of light) are passed, or when there is ithasala (planets moving toward an exact aspect). When there is no aspect, there is neither ithasala nor israpha. Then gain of a son is told from the birth chart.

Simple meaning: Example, a son: Jupiter as 5th lord promises a son at birth. If the year lord and year-lagna lord move apart from him in his period year, no son that year; otherwise the birth promise holds.