prop up, support, brace
撑 centers on holding something up so it stays in place: propping up, supporting, or bracing against collapse.
撑 is a phono-semantic compound: the hand radical 扌 indicates the action of propping or supporting, while 掌 (which itself means palm or control) likely contributes the sound. The exact historical development is uncertain, but the component relationship is clear.
The hand radical 扌 beside 掌 (palm) suggests a hand pressing upward with the palm to prop something up. Picture bracing a heavy shelf with your palm to keep it from falling.
For チョウ, imagine a long-serving chief named Chou propping up a sagging beam with his palm: Chou -> チョウ, and his hand supports the weight.