Jerking question

I saw a lot of movies here with jerking cock. Masturbating. Can anyone explain to me why this happens? You jerk your cock fast and then you slow a bit.... then again go fast... and then again slow down for a second or two. And after a few times you cum. I realized I behave exactly like that, I can't help myself... I simply need to slow it down and then fast jerk again... and so on until cum.