Assembly for an air circuit of a heat engine, comprising: - a first duct which can convey fluid; - a second duct which extends between an input in the first duct and an output in the first duct, such as to form a bypass of a portion of the first duct, the second duct comprising a source of pressure variation; and - a system for routing of the fluid into one out of the second duct and said portion of the first duct, the routing system having a first configuration which allows the fluid to circulate mostly in said portion of the first duct, the routing system comprising: - a retention unit which exerts a torque configured to bring said routing system into the first configuration or retain it there; and - at least one out of: an area (2, 3) which blocks in this first configuration all or part of the input of the second duct, and an area (2, 3) which blocks in this first configuration all or part of the output of the second duct, the routing system being designed to go into a second configuration which allows the fluid to circulate mostly in the second duct when the pressure variation generated in the second duct by the source exceeds a predefined value, this pressure variation then exerting on said area(s) (2, 3) of the routing system a torque which permits this passage into the second configuration, despite the torque exerted by the retention unit, the retention unit being such that the torque that it exerts on the routing system decreases when the routing system passes from the first configuration to the second. - a first duct which can convey fluid; - a second duct which extends between an input in the first duct and an output in the first duct, such as to form a bypass of a portion of the first duct, the second duct comprising a source of pressure variation; and - a system for routing of the fluid into one out of the second duct and said portion of the first duct, the routing system having a first configuration which allows the fluid to circulate mostly in said portion of the first duct, the routing system comprising: - a retention unit which exerts a torque configured to bring said routing system into the first configuration or retain it there; and - at least one out of: an area (2, 3) which blocks in this first configuration all or part of the input of the second duct, and an area (2, 3) which blocks in this first configuration all or part of the output of the second duct, the routing system being designed to go into a second configuration which allows the fluid to circulate mostly in the second duct when the pressure variation generated in the second duct by the source exceeds a predefined value, this pressure variation then exerting on said area(s) (2, 3) of the routing system a torque which permits this passage into the second configuration, despite the torque exerted by the retention unit, the retention unit being such that the torque that it exerts on the routing system decreases when the routing system passes from the first configuration to the second. - a retention unit which exerts a torque configured to bring said routing system into the first configuration or retain it there; and - at least one out of: an area (2, 3) which blocks in this first configuration all or part of the input of the second duct, and an area (2, 3) which blocks in this first configuration all or part of the output of the second duct,