My understanding is that it's when it comes forward so that it is ready to stream out faster. If you are leaking you probably let down, but you can also let down without leaking. But it's very obvious when you are pumping (tons more milk suddenly) or BFing (slower, rhythmic sucking with more swallowing).
Doesn't matter how your supply is the sensation is when the milk is released from the ducts all at once. I feel mine very strongly when I'm not drinking enough water. If I'm hydrated I barely notice it
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