练 = 纟 + 柬. The silk radical 纟 (sī, the abbreviated form of 糸 mì) sits on the left and carries the meaning; the phonetic 柬 (jiǎn) sits on the right and once carried the sound. This is a clean pictophonetic build: thread on one side, a sound-marker on the other, the two welded into one craft-word.

The original sense is physical and wet. Raw silk reeled off the cocoon comes off stiff, gummed, and yellowish with sericin. To make it usable you boil it, again and again, in alkaline water until the gum lifts and the fiber turns soft, white, and pliant. That repeated boiling-and-rinsing of silk is the root meaning of 练: 熟丝 shú sī, cooked silk, as opposed to 生丝 shēng sī, raw silk. The Shuōwén Jiězì glosses it squarely as treating silk.

From the steaming vat the meaning climbed off the thread and onto the person. What you do to raw silk, you also do to a raw skill: subject it to the same treatment over and over until the stiffness boils out and it runs smooth. So 练 became to drill, to practice, to train, and the participle 练 came to mean tempered, seasoned, experienced. A person who is 老练 (lǎoliàn) has been through the vat many times.

Japanese keeps the thread sense vividly in 練 (ren): 練る neru is to knead, to refine, to work dough or metal or a plan until it is smooth, the same image of repeated working that softens and strengthens.

When 练 stops being the action and becomes the result, it names the quality the drilling produced: practiced, fluent, seasoned. 熟练 is smooth-from-repetition; 老练 is veteran, weathered by long experience; 教练 turns the trainer into a noun. The thread that came out of the vat soft and white is now the hand that moves without hesitation.

Picture a dye-house courtyard. A worker lifts a hank of raw silk, stiff and yellow, out of a cauldron of boiling alkaline water, dunks it back, lifts it again. Steam, the smell of wood ash, the same motion a hundred times. With each cycle the thread loses a little of its gum and gains a little whiteness, until what came out brittle goes back in soft. That cauldron is the left side of the character, the silk radical 纟; the phonetic 柬 on the right is just the bystander who tells you it sounds like jiǎn.

Now move the cauldron indoors. Swap the silk for a student at a desk, a runner on a track, a recruit on a parade ground. The treatment is identical: the same motion, repeated, until the rawness boils off. 练习 is the dunking. 训练 is a foreman running the vat to a standard. 熟练 is the finished hank, smooth and white. 老练 is the silk so often boiled it will never go stiff again.