The children will fill up the dump trucks, drive them to the edge of the ramp, and then dump. Did you notice there are four sets of hands in this small space?
One thing to note that is highlighted in this video is that a channel, once it is cleared, is like a track that directs a back-and-forth motion without having to worry about running off the track. For this child, it allows him to make those big motions, which in turn, frees up his focus so he can also do something else---like sing. Those big motions must be so liberating.
When playing with this apparatus, some children go so far as to empty all the channels into the tub. When the older children put their mind to it, they can do an amazing job. But how do you get the last little bit out of those pesky cracks? You use what you have: your fingers.