In vert kicking, the pop transition is mostly used for backside takeoffs, as frontside pop kicks are relatively rare anymore, aside from the occasional combo-starting pop kick.
From either stance, the easiest, most natural movement into the pop is often a round kick trick. Usually, this means kicking through the target all the way to backside, or landin hyper-style for a faster transition. Into frontside pops, this means kicking through the target, and setting it down no more than 90 degrees past the target, essentially in an outside stance. The round kick trick setup is convenient because it mimics a step into the trick, which is very natural feeling.
Conversely, hook tricks into pop kicks tend to be somewhat less natural for frontside pop kicks, and often become bound transitions, whether the performer realizes it or not. In this scenario, the hook kick is often set down in an inside stance, then the inside leg steps in, or across, into the pop takeoff. It is this adjustment step that differentiates a true pop transition from the unified transition family’s secondary form, the bound. In order to do a true pop into a frontside vert kick, the hook needs to be put down in frontside, and an immediate takeoff from both feet needs to occur. Notice in the clip above, the pop is performed, but there is a slight adjustment in the feet that could technically be argued to create a bound. It seems most reasonable to take the transition by its intent, and not focus on minor imperfections.
Utilizing similar technique, but without the extra step, hook tricks into backside pop kicks often takeoff from closer to an inside stance, and have approximately 90 degrees less rotation than their true backside versions, but are still recognized as backside tricks. This is similar to cartwheels and other inverted tricks that land hyper followed by pop tricks.