But Onan knew that the seed would not be his; and it came to pass, when he went in to his brother’s wife, that he spilled it on the ground, so as not to give seed to his brother.
English: Similar to the act of Er and Onan, as it is written: “And it was, when he came in to his brother’s wife, that he spilled it on the ground.”
On the daf / in context: Yevamos 34b identifies Onan’s act from the verse’s language “spilled it on the ground,” and links Er’s death to the same pattern.
Type of Learning
Derash on the verse (Bavli sugya) — not a formal named middah of the 13. The extra wording “on the ground” specifies the nature of the act.
What Is Learned
Onan deliberately wasted seed so as not to establish offspring for his brother; the Torah marks this as the sin for which he died.