A boiling cauldron has a rising, trembling surface and often a continuing roar or bubbling sound. The poet selects this familiar experience to express the horse's inner stored energy. Ghalyu mirjal supplies not a static anatomical image but a dynamic and almost audible one.
The first part, ʿalā al-dhabl, shows a relatively lean outline; the second, jayyāsh ... ghalyu mirjal, depicts vigor bursting through that outline. External leanness and internal force therefore contrast meaningfully.
Earlier, the horse moved with rock-like momentum. Here it moves with cauldron-like turbulence. These similes do not merely repeat “fast”; each depicts a different dimension of power.