Non-interlaced NTSC

[lnx64]

Is there anyone that can explain how exactly non-interlaced NTSC works? I've heard a few reasons for it.

One is the last scanline is only half a scanline in interlaced modes, and in non interlaced modes, the last scanline is a full scanline.

But then I've heard others where the VSync has extra pulses to prevent the two fields from advancing basically.

So what exactly causes interlacing and no interlacing?