The amazing photo in the header post illustrates are are times in the running cycle when neither of a runner's feet are touching the ground.
Back in the 1800s, people argued whether there was a point in the running cycle of a horse when none of the four hoofs touched the ground.
Some argued it was not possible for a 2-ton animal to be airborne. Leland Stanford, oil barron, founder of Stanford University, and Governor of California, argued to the contrary. He solicited the help of one Eadweard Muybridge (born Edward James Muggeridge), who proved his point with a photo (since lost) in 1877 and followed by a full studio of the running cycle of the horse.
To understand how it is possible for a 2-ton horse or a roughly 1/8 ton person to be airborne at some point when at a full gate, you have to understand some engineering principles, which we'll not delve into here.