Based on the characteristic hemodynamic changes, the Valsalva maneuver divides into four phases.

Phase I, which corresponds to the onset of strain, is associated with a transient rise in blood pressure because of the emptying of some blood from the large veins and pulmonary circulation.

Phase II follows this when positive intrathoracic pressure leads to a reduced venous return to the heart. Because of reduced venous return and thus reduced preload, stroke volume falls; this leads to a fall in blood pressure activating the baroreceptors in the carotid sinus and aortic arch. The vagal withdrawal followed by increased sympathetic discharge ensues, leading to marked tachycardia, increased cardiac output, and vasoconstriction, which leads to the recovery of blood pressure to normal values in healthy individuals.

Phase III is the transient phase involving the release of strain which leads to a sudden dip in blood pressure. The release of positive pressure leads to expansion of the pulmonary vascular bed and reduces the left ventricular cross-sectional area resulting in a transient fall in blood pressure.[6]

Phase IV is the overshoot of the blood pressure above the baseline, which is because of the resumption of normal venous return to the heart stimulated by the sympathetic nervous system during Phase II. The overshoot of blood pressure leads to stimulation of baroreflex, leading to bradycardia and the return of blood pressure to the baseline

https://www.ncbi.nlm.nih.gov/books/NBK537248/

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