See the original markdown specification (bold mine):
The implication of the “one or more consecutive lines of text” rule is that Markdown supports “hard-wrapped” text paragraphs. This differs significantly from most other text-to-HTML formatters (including Movable Type’s “Convert Line Breaks” option) which translate every line break character in a paragraph into a
When you do want to insert a
<br />break tag using Markdown, you end a line with two or more spaces, then type return.