Returns the channel name that will be used for looking up
existing animations for a given target. Effects that are on the 'null'
channel are not tracked in this way. Otherwise, when the effect is
run, the channel is used to provide information to the effect about
any previous effects that were run. This allows the new effect
to adjust its own animations accordingly, either reducing time,
starting from a different point, or entirely custom behavior depending
on the needs of the effect.