The wedge is for a tilted cake. The one I'm doing will be similar to the pic by paolacaracas, but with a "Construction" theme.
For my topsys, I just stack them like a normal tiered cake, putting a small dab of piping gel to help them stay in place and hammering 2 dowels through the entire cake and into the foam core base once it is stacked.
If the dowels aren't long enough to leave some on top to trim, or if there won't be a topper or other decor to hide the holes, then it's a bit more trciky as you have to cut holes in the cake board of the top tier to slide over the dowels.
For that, I hammer the dowels into all tiers except the top, leaving an inch or more protruding. Then I dab food color on the tips of the dowels to mark the areas where I'll have to make holes (on the underside of the top tier). It's precarious to make the holes since you have to do it from the underside, but it's not hard. Then you have to make sure that you are holding the top tier at the same angle as the tier below it so that when you slide it over the dowels it sits flush on top of the tier below.
I have never had a problem doing this, even with a raspberry filling that sits on top of a cheesecake filling (I dam it well!).
The old way was much more stressful for me, trying to make that hole just the right size so that the tier resting on the flat spot doesn't crack the dam or leave a gap that must be covered with a big border.
I saw Lindy do it in her book "Cakes to Inspire and Desire", so I I gave it a shot and never looked back.
The pink cake was huge and I delivered it fully assembled without issue. (I deliver all of my topsys assembled, but they have all been 2-tiered).