Peah 6:3-4

Peah 6:3

If a sheaf is at the head of a row, the sheaf adjacent to it proves that it is not shich’cha (i.e., since the adjacent sheaf was not yet gathered, we assume that the owner intends to gather the sheaf in question at the same time). If a sheaf was taken with the intention to bring it into town and was subsequently forgotten, Beis Hillel and Beis Shammai agree that it is not shich’cha (because the owner has already taken possession of it).

Peah 6:4

The heads of the rows are as follows: two people start gathering in the middle of a row facing the edges of the field. If they forgot sheaves both in front of them and behind them, those in front of them are shich’cha and those behind them (meaning in between where the two people started from) are not shich’cha (because each assumed the other would gather it). If one person started gathering at the edge of the field working towards the other side and he forgot sheaves both in front of him and behind him, those in front of him are not shich’cha (because he hasn’t passed them by yet) and those behind him are shich’cha (because he’s not allowed to turn back for them). The general rule is that if collecting a sheaf would violate “you shall not turn back” (Deuteronomy 24:19) it is shich’cha and if collecting it would not violate “you shall not turn back,” it is not shich’cha.