tighten, stiffen
斂 centers on drawing something tight or making it rigid: tightening a grip, stiffening a posture, or pulling things together firmly.
斂 combines 僉 (a component associated with gathering or all) and 攵 (a hand holding a stick, suggesting action). The character likely expresses the idea of pulling or drawing together, leading to meanings like tighten or stiffen.
The left side 僉 looks like a group of people gathered under a roof, and the right side 攵 is a hand with a stick. Imagine using a stick to pull a rope tight around the group, making everyone stiffen and stand rigid.
For レン, picture a wrench tightening a bolt: wrench -> レン, and the action makes the joint stiff and secure.