When after cumming into a someones mouth you ask them to "pass it back", the sperm is then transferred from the original recipient's mouth to it's original owners mouth by any means. After this takes place the sperm is then swallowed by it's maker.
Your girlfriend is giving you head, and you cum into her mouth; however, you ask her not to swallow, but to instead, pass it back, put your own sperm into your moth so you can swallow it.
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