Invisible moonkeys pushing them in. And some additional fuckery, which basically involves taking HD pressure morphs from a different asset and making the sweater fit to it. The other way around - making the morph yourself will work and give you a good result only if the point of pressure is aligned well with the geometry of base mesh of the figure, because boys at Daz said fuck you and you can't make morphs on anything above subd0 unless you are their vendor.
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A quick rundown: I start with my figure with big booba body, first I load my
correction morph on the clothes, cause of course I do. If you have a d force piece of clothing that can be used to fit the figure you can bypass that. (If you want to save the simulation as a morph
here we discussed that a while back.)
Next I load my push up morph (Cleavage T3), this is simply just morph with boobs rotated and push upwards, there probably are some similar assets, just not for boobs the size I use, or if there are any, I couldn't be bothered, because they are extremely easy to make just with sliders without even going into modeling software.
After or before I dial up the pressure morph to infinity and beyond, but you got to turn off the limits first. (By clicking the gear icon next to it and in the popup window next to set limits checking that off.) /sidenote anything past certain point will start breaking the mesh, so good value is somewhere between 200-400
Then I dial up sweater cleavage morph on the clothing that I made specifically to fit the pressure morph, but it can be pretty much just used as a cleavage morph if anything.
And then I fuck around with additional dials on the clothing and from fit control if necessary to make the sweater fit better and I up the mesh smoothing. It's often really annoying to avoid any pokethroughs and it's much more time efficient to just fix it in photoshop after rendering in my experience.
That's pretty much it.