It adds an extra twist when tying the first throw, forming a double overhand knot.

The additional turn provides more friction and can reduce loosening while the second half of the knot is tied.

[1] This knot is commonly used by surgeons in situations where it is important to maintain tension on a suture, giving it its name.

[2] Surgeon's knots are also used in fly fishing, in tying quilts, and for tying knots with twine; it is particularly useful in tying raw meat with butcher's twine, as the wet meat creates similar risks of loosening as surgery.

Some sources categorize the surgeon's knot as a bend, since it can be effective as such.