Designing for the tool, not the shape
How the process constrains the geometry before anything is drawn.
Designing for the tool means starting from what a chosen manufacturing process is physically capable of producing and shaping the part to fit within that, rather than drawing the ideal shape first and hoping some process can be found to make it, because a shape drawn without any process in mind very often turns out to need a movement or an access that no single tool can actually provide.
A cookie cutter can only go down and come back up
A cookie cutter presses straight down through rolled dough and lifts straight back out, and that single motion can produce any outline it likes (stars, rings, animals with thin legs) but never a shape with dough tucked underneath a bridge or sealed inside a cavity, because no straight-line motion could remove the cutter from a space like that once it was pressed in. A baker wanting a biscuit shaped like a hollow ball would find that no cookie cutter could make it. The shape is easy to imagine; what it needs is an access the cutter's one simple motion cannot provide. Every manufacturing process is a cookie cutter with its own allowed motion, and a shape has to respect that motion before it can be made at all.
Each process has its own forbidden shapes
Every manufacturing process can only move material, or move a tool relative to material, in certain ways, and those allowed movements set a hard boundary around what shapes the process can produce, however the desired shape is drawn. A process that only presses straight down and lifts straight back up can never produce material hanging over open space beneath it, an undercut, because no straight up-and-down motion could have got the tool in or the material out. Injection moulding is one of the clearest examples, since a mould that opens in a single straight direction can only release shapes that respect that one direction of travel, however complicated their outline is otherwise. A rotating tool, a drill or a lathe, carries its own equally strict version of the rule, since anything it produces has to be reachable by something spinning about a fixed axis, which rules out just as many shapes as a straight-pull mould, only different ones.
Designing for the tool means checking a shape against this kind of constraint before committing to it. The useful question is whether a straight line, a rotation, or whatever simple motion this particular process is built around could actually produce each feature, and anything that fails the test gets reshaped on paper, long before a tool has been built to make it.
The check has to cover every feature, including the small ones. A single pocket or overhang a few millimetres across, in an otherwise straightforward part, can rule out a whole class of process on its own, turning what would have been a quick single-motion operation into one that needs a slower and more expensive approach, such as a mould with extra sliding sections or a second machining setup. The feature that ends up ruling out a process is rarely the one that looked like trouble at first glance, so checking only the awkward-looking features misses it.
Two halves and a hole opened up
Because the constraint comes from the tool's motion and has nothing to do with what the part is for, a shape can very often be redesigned to do the same job while fitting within what a simpler process allows. The baker who wanted a hollow ball badly enough could still get close by baking two dome-shaped halves with an ordinary cutter and joining them afterwards, and a designer reaches for exactly the same workaround, splitting an undercut feature into two pieces assembled afterwards. Another common fix is to open a pocket up at one side so the tool can reach it in a straight line. Finding that redesign early is far cheaper than discovering the shape cannot be made after it has been drawn, detailed and quoted, and checking a design against the tool before it is finished turns an expensive late surprise into an ordinary early adjustment.