Short answers, pulled from the story.
Dick Huemer developed a system for inbetweening during the 1920s that changed how animation studios operated. This method relied on a light table to draw pencil and paper sketches with precision.
Fantasmagorie released in 1908 stands as widely considered the first fully animated movie using frame rates on twos. Most movements require only twelve drawings per second rather than twenty-four to maintain the illusion of motion.
Adobe Flash emerged in the late 1990s as one of the earliest software programs utilizing automatic interpolation. Cambridge Animation Systems developed Animo around the same time to handle sophisticated digital transitions.
Nestor Burtnyk and Marceli Wein worked at the National Research Council of Canada during the 1990s. Their collaboration earned them a Technical Achievement Academy Award in 1997 for their contributions.
An ease-in transition starts an animation slowly before progressively getting faster as it continues. These parameters define the physics of the transition between two distinct animation states.