According to the Torah, when a farmer harvested his field, he was obligated to leave one corner unharvested, so that the poor could harvest it themselves and keep the produce. When gathering sheaves together, any sheaves that she forgot or stalks that were dropped are left for the poor as well.
The last two laws — shikchah (forgetting a sheaf) and leket (dropping them) — are curious because they are mitzvot that one cannot do deliberately. If you pretend to forget, you have not performed the mitzvah; if you drop it deliberately, you may still leave it for the poor, but you haven’t fulfilled the mitzvah of leket, which requires inadvertence.
There is a way to make it likelier that you will fulfill the mitzvah, however. If you feel a sense of surplus, of plentitude — that God provides more than you need — and if you harvest with the slightly careless attitude of one reaping great riches rather than one counting few coins, you are more likely to forget or drop some of the harvest. Living in gratitude makes one less frugal and less stingy. So gather with confidence in life’s immeasurable abundance, and others will find blessing in what you leave behind.