It is heard following the middiastolic rumble of the stenotic valve,[1] during the diastasis phase, making it a "late diastolic" murmur.
The murmur is heard due to antegrade flow of blood through a progressively narrowing mitral opening during the end of the atrial systole.
This antegrade flow through the mitral valve before it completely closes appears to be the result of a pressure gradient at the end of diastole.
[1] As its name so indicates, the presystolic murmur is heard before the mitral valve produces the S1 heart sound.
Less often, a presystolic murmur can be heard when a right atrial myxoma causes a tricuspid valve obstruction to blood flow.