Well any Track has a 'direction' - ie it starts at the first trackpoint listed and ends at the last one - and the old Vista and Legend types didn't really recognise this, hence you had to choose which way you wanted to travel along the Track when using Trackback. You could of course just colour the Track a nice bright colour and not bother with Trackback at all, you can still follow it just the same, and in either direction.
You can do that on the new models too, though because the map rendering is a bit heavier, the coloured Track doesn't stand out so well on the screen as it did on the old models. (The 30x with its higher-resolution screen may be better - or worse - than the 30 in this regard.) And, unlike the older models, by default Track displays are turned OFF - so to see a Track in this way you have to go into the menus to 'Show' it (not to be confused with 'See on map' - which is quite a different thing).
But otherwise the wiggly purple line mode only makes sense if you travel along the Track in the right direction. (Here I dig out my E30 to check something, which I don't use much, preferring my old HCx - now where is it - ah, found it, wrapped in a glove, in a shoe, in a touring pannier - it's my travelling spare.) So you have an option in the menus - in Track Manager - to 'Copy reversed' a Track. This is the new equivalent to picking which end to start from in Trackback.