Flexible member for resisting retrograde flow

A method comprising:

• (a) inserting a device into a lumen, the device comprising:
• (i) an outer elongated member,
• (ii) an inner elongated member disposed within the outer elongated member and axially movable with respect to the outer elongated member,
• (iii) a stop member disposed about the inner elongated member and axially movable with respect to the outer elongated member, and
• (iv) a flexible structure having a distal portion attached to the stop member and a proximal portion attached to the outer elongated member and configured such that when the inner elongated member is extended distally relative to the outer elongated member, the flexible structure adopts a reduced width profile and such that when the inner elongated member is retracted proximally relative to the outer elongated member to a point of maximal retraction, the flexible structure adopts an expanded width profile that is more resistant to deformation due to fluid pressure in a proximal direction than it is to deformation due to fluid pressure in a distal direction, said device being inserted into the lumen while the flexible structure is in the reduced width profile,
• (b) retracting the inner elongated member proximally relative to the outer elongated member to the point of maximal retraction, such that the flexible structure adopts the expanded width profile with a first frustum having a first frustum angle substantially within a second frustum having a second frustum angle that is smaller than the first frustum angle, the expanded width profile engaging an inner surface of the lumen and creating a check valve in which the expanded width profile is more resistant to deformation due to fluid pressure in the proximal direction than it is to deformation due to fluid pressure in the distal direction.