But why, for he is lessening the four Amos - ואמאי והא קא ממעט מארבע אמות
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
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.