The Infusion Nurses Society (INS) deals with self-sealing and needle-free connectors in its guidelines for practice in the Infusion Therapy Standards of Practice (published in 2011 by the American Journal of Nursing). The primary purpose of the needle-free connector is to protect health care personnel from the needlestick risks in intermittent intravenous administrations, as well as to prevent infections in patients.
Internally, the dynamics of the operation of the needleless connectors is related to the displacement of the fluid, which can be negative, neutral or positive. To specify a little more, it is these internal mechanisms of fluid displacement that impact blood reflux, (see the article Choosing the Best Design for Introvenous Needless Connectors to Prevent Bloodstream Infections, 2010). However, a note from INS is important: when the need is a high flow rate, the connectors should not be used, as they reduce it.
Understanding the difference between flows: