practice, gloss, train, drill, polish, refine
練 centers on repeated effort to improve: practicing a skill, training, drilling, or refining something until it becomes polished.
練 combines 糸 (thread) with 柬, which likely contributes the sound and the idea of selecting or sorting. The original sense may relate to refining raw silk by repeated processing, which extended to practice and training.
The left side is thread 糸, and the right side 柬 looks like a bundle being sorted. Picture repeatedly working raw silk threads until they become smooth and polished—that is practice and refinement.
For レン, imagine a long training session where you practice writing the kana れん over and over: れん -> レン, and the repetition polishes your skill.
practice; training; drill; (an) exercise; workout
training; drill; practice; discipline
to knead; to thicken into a paste (stirring over a flame)
test; trial; ordeal; tribulation
refinement; polish
skill; expertise; experience; proficiency; mastery; dexterity
lingering attachment; lingering affection; regret; reluctance; ruefulness
training; drill; practice; practising; discipline
kneading; gloss; tempering
condensed milk
morning training (e.g. before school)
to parade; to march; to walk in procession
briquette (charcoal or coal)
paste products (incl. sweets, fish paste, etc.)
skill; dexterity; master hand
means of deceiving another
drill
experienced; veteran; skilled; expert
skilled worker; skilled labour
training; practice; drilling; cultivation; improvement