paralysis, numbness of limbs
瘓 centers on a loss of physical function: paralysis or numbness in the limbs. The meanings are closely related, describing a state where movement or sensation is impaired.
瘓 combines 疒 (sickness) with 奐, which likely contributes the sound. The exact historical development of 奐 is uncertain, but the character as a whole is a phono-semantic compound for paralysis.
The sickness enclosure 疒 wraps around 奐, which looks like a person with outstretched limbs. Imagine a sickness that freezes those limbs, causing paralysis and numbness.
For タン, picture a person with paralysis getting a tan from lying motionless in the sun: tan -> タン, and the stillness of the limbs matches the meaning.