Explanation of the author's statement: "…and what is combined with it before it…"

The Shaykh : But he said "and what is combined with it before it". For example, if a person catches a portion of the 'Asr prayer, such as one rak'ah or the amount of the opening takbir, then the 'Asr prayer becomes obligatory upon him, and the Zuhr prayer also becomes obligatory upon him. If he catches that amount in the 'Isha' prayer, then the 'Isha' prayer and the Maghrib prayer also become obligatory upon him. If he catches that amount in the Fajr prayer, then only the Fajr prayer is obligatory upon him, because it is not combined with the prayer before it. The ruling for Fajr is clear: he is not obligated to pray anything other than it. So what remains is the case where a person catches a portion of the time of 'Asr or the time of 'Isha'. It is clear that the 'Asr prayer is obligatory upon him because he caught its time, and it is clear that the 'Isha' prayer is also obligatory because he caught a part of its time. But what is the reason for the obligation of the Zuhr prayer in the first case and the Maghrib prayer in the second case? They say that the reason for the obligation is the athar (report) and reasoning. As for the athar, it is narrated from Ibn 'Abbas and Abu Hurayrah (may Allah be pleased with them). As for the reasoning, it is because the time of the second prayer is a time for the first prayer in the case of an excuse. The excuse that permits combining the prayers, when it is a time for the first prayer in the case of an excuse, makes catching it, or catching a part of it, like catching a part of both times. Do you understand? Good. And this is the well-known position in the madhhab (school of jurisprudence).