It something learned by experience and knowing how the plant grows.. Micranthemum spp. and Hemianthus callitrichoides require the same approach.
The most important one of course is sufficient CO², Ferts and Light. Than if it has this it will grow very dense, and this will suffocate the leaves at the lower stems at the substrate, depriving it from light etc.. At one point this will all be melted away leaving you with nacked stems close to the substrate. At that point you are already to late with trimming it close to the substrate. Because than you will have about 0 foilage left, than replanting cuttings is the only option which is an absolute pain in the neck.
If you wait even longer than even the stems at the substrate will melt and the plant will be no longer rooted. Since it gets ampple co² and ferts via the water column, the plant has no need to search for it in the substrate, it gets all it needs with hoovering above it with also roots free floating (air roots?). Than you can find an intire carpet comming up to the surface floating one day.
The tricky part is, the plant needs to be trimmed when looking at its best.. That the point looking so nice you want it to look like that for ever. Than not trimming it that melt at substrate level will kick in withing 1 or 2 weeks. And because it grows rather fast if given the perfect conditions it requires almost constant maintenance.. Depending on how perfect the conditions are maybe about every other day trimming here and trimming there.
Or neglect it for a week or 2 enjoying its lovely looks, than vigorously trim it back wait 2 weeks for it to recover and back to square one going over and over again.
Having a gorgious short MC or HC carpet is simply a lot of delicate work trimming and pressing it down since it is such a delicate tiny plant.
I guess having that constant perfect HC carpet it something typical for Traditional Eastern Aqauscapers living according the Wabi Sabi philosophy.. Working on it every day with heart and soul and feel it is a theraphy instead of a repetitive nuisance.
I personaly belong to the later group always posponing trimming it till tomorow, that's why i gave up on trying to adchieve that. This plant sp. is to labor intensive. Than MC is a tad easier than HC.