It is a method of using some of the spare lines in a TV picture to give a suitable TV enough additional info to be able to generate a true 16x9 picture (aka anamorphic) from a 4x3 one. It was (is) used in the UK only by Channel 4.
I actually wanted to know exact technical description what sort of modulation, coding or whatsoever in the picture is done in Pal Plus. Or are simply more lines transmitted (ev. difference in pal signal between 625 and 576 lines)