The alpha1 announcement refers to a new Monotonic infill option. I see the option in the Top and Bottom Infill Pattern parameters in the alpha3 release, but I don't see the kind of effect the announcement describes in the sliced image on the plater window.
What should I be looking for?
It provides a more even top surface. Print something with a flat top surface with holes in it to compare. The rectilinear pattern attempts to fill in the top are precisely. It will create even rows, but eventually will have to extruder the odd wide or narrow line to fit. The "lawnmower" effect causes alternating rows to look nice, but an odd row will stand out. They can also be over-extruded slightly.
Monotonic emphasizes even travel, keeping a consistent pattern across the surface. From what I've observed, it does this partially by breaking the rules to allow an extrusion to cross a perimeter around a hole, for example.
Yes, I can certainly see the difference in the photo above, as well as the photo on the releases page in github.
I was only looking at the slicer image that shows the placement of the filament lines - the two patterns look identical. But the description led me to understand each line was laid down in the same direction, like left to right. But that doesn't appear to be the case - the lines are laid down back and forth.
Now I see it. Moving the slider bar at the bottom of the page shows the order that each row is laid down - in the rectilinear pattern, it lays the rows down progressing to the left and right; but in the monotonic pattern, it always lays them down going in the same direction.