Thats what is known as hand propping. The cylinders of the plane are run through their cycle a few times to make sure the oil isn’t settled at the bottom, by hand turning the engine; this allows the engine to turn freely as possible. The propellor is then turned so that the cylinders are top dead center, which means that the air is compressed in the cylinders, theres fuel ready to be ignited and the spark from the ignition will occur as soon as the cylinder passes through top dead center.

When you hear the crew yell “contact!” The pilot usually responds by checking the breaks and activating the ignition system and replying contact. Ground grew then swing the prop through (theres a whole technique to this so that you don’t lose an arm) thus sending a spark through the system as it passes through top dead center. Now if you’re lucky the fuel/air mixture will ignite, thus pushing the cylinder through and causing other cylinders to ignite, and the whole sequence become self sustaining. (Look up Otto Cycle engine.)

This was common in the early days of aviation. But as the number of cylinders increased, the compression ratios got higher, the engines and props got bigger, and fuel injection came along, it rapidly fell out of favor. The technique can be used in a few modern airplanes, but it is rare these days.

I can source this later for you. But I have a degree in aeronautical science and this is some of the stuff from aviation history classes, if that lends credibility.