i just cut them where a set of leaves are growing cabomba for instance i just cut it above the leaves and put it in another tank (which i use to grow my plant they grow like mad in there lol) and within a week or two they are rooting well even when they are floating in there too its alot easier to root aquatic plants ive found. with swords i just snap a full stem and leaf from the vey bottom of the plant and they root and multiply quite well ive found too
Cuttings:
Many aquarium plants bread by cuttings where a growing part of a stem is planted. With a clean cutting cut down a part even 30 cm long. Many plants hardly root if they swim in the water or they are lie on the bottom. Steams usually root from the ganglion on the steam. But on some plants they can grow from any part. Typical plants which breed by cuttings are: Bacopa caroliniana, Cabomba caroliniana, Egeria densa, Hygrophyla polysperma, Limnophila aquatica, Ludwigia mullertii, Myriophyllum hippuroides and Hygrophila difformis.
Lateral roots:
Some favourite kinds of plants makes lateral rhizomes, little plants growing from the mother plant. For example echinodoruses create new plants from ganglions based on the leave rosette, while the Indian moss grows filial plants from the border of leaves which can separate and swim on the surface. But if you cut them down, you can plant them. Vesicularia dubyana (The Java moss) creates a huge bunch of little plants separating from the mother plant and catching on any near surface one.