The "override" setting in a timeline's Properties does the same that as that of a button: It temporiraly changes the end action of the timeline it links to.
Timeline B has an end action that is set to link to timeline C.
Timeline C has an end action that is set to link to timeline D. And timeline D's end action returns to the menu.
Timeline A has its override set to "return to menu" and an end action linked to timeline B.
Button X is linked to timeline B, button Y is linked to timeline A.
If button X is used, timeline B will play, followed by timeline C, timeline D and then the menu re-appears.
If button Y is used, timeline A will play, followed by timeline B and finally the DVD will go back to the menu.
The main use of the "override" option is to create a "play all/play separate" menu. For this the "override" is set in the Properties of the buttons, not the timelines themselves. In Encore 1.5 the same thing is easier achieved by using its Playlist feature.
The "override" in the Properties of a timeline is only used for special navigational tricks.