But why, for he is lessening the four Amos - ואמאי והא קא ממעט מארבע אמות
Summary:
According to רש"י, the case of the דבילה is where he threw it exactly ד"א. However תוספות maintains that he threw it more than ד"א; however the מקום הדבילה is not a רה"ר (since if he threw a needle on it he would be פטור).
He threw a board, etc. The landing of the object and making the partition, etc. - זרק דף כולי הנחת חפץ ועשיית מחיצה כולי
Summary:
The case here is where there are two walls formed by the יתידות and the third wall is formed by פי תקרה. A רשות which is דע"ד on top but not דע"ד below; the top is considered a רה"י, but not the bottom.