tighten
歛 centers on drawing something in or making it tighter, like pulling strings together or constricting an opening.
歛 combines 僉 (a phonetic element with a sense of gathering or all together) and 欠 (open mouth or action), suggesting the act of drawing in or tightening. The exact historical development is uncertain.
The left side 僉 looks like a gathering of people under a roof, and the right side 欠 is a person with an open mouth. Imagine everyone pulling in their breath together, tightening the air in the room.
For レン, picture a wrench tightening a bolt: wrench -> レン, and the tool pulls everything tight.