How do sperm cells move up during ejaculation, is it because of stimulation and then at a certain point it gets released?

When the sexual stimulus becomes intense, the reflex centers of spinal cord emit sympathetic impulse to initiate emission resulting in expulsion of sperm into the internal urethra followed by the contractions of prostate gland and seminal vesicles expel prostatic and seminal fluid into the urethra. Finaly, increases in pressure in erectile tissues in penis and genital ducts ejaculates semen out.

The sperms stored in the seminal vesicle along with fluid from the prostate are propelled out by their contraction and the muscles surrounding the base of the penis