It interactively shows you each change that would be added and lets you decide whether it should be staged or not. Down to the hunk level, so you can partially stage a file if you so choose.

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eterm 4 hours ago | parent [-]

Oh, that's neat.

The -p presumably stands for pInteractive with a silent p :)

	▲	dxdm 7 minutes ago \| parent \| next [-]
		There's -i for interactive, lets you add, patch and reset (unstage).
	▲	oarmstrong an hour ago \| parent \| prev \| next [-]
		Ha! It’s actually patch (as in select parts of the patch) though I remember it as partial.
	▲	airstrike 3 hours ago \| parent \| prev [-]
		LOL it might stand for "prompt before"