When to engage/close the bypass

[stoveliker]

It's like this.
A valve on a T that opens up once the suction (draft) is too high, letting in air from the room in which it is in.
Generally their trigger point setting can be set by moving a weight. (note that this one seems to be upside down...)
So they resolve high draft on the stove by satisfying the suction partly by flow through the stove and partly by letting in air from the room.

 

[Tron]

Ah, so they don't control the flue diameter with a physical damper but let air in to relieve suction pressure. Understood.