Add or Remove Space Before a Paragraph

Paragraph spacing is the amount of space before and after a paragraph. This can be specified in the paragraph settings (Shift + F10, then P).
But a quick way of adding 12 pt spacing before a paragraph is to press Control + 0 (zero). Press again to remove the spacing.
If there is more spacing before a paragraph, Control + 0 will bring the space back to 12 pt.
Pt or Point is a measure of font size and 12 pt is just over 4mm.