This is wrong information. There’s no difference between holding down Spirit Barrage, or rapidly clicking. It’s not a channeling skill, so either way you’ll get as many casts that your attack speed allows. A phantasm will explosion for each cast after 3 are on the ground.
Also there is no interrupting necessary. You don’t need to use another skill. Simply stopping to cast Spirit Barrage means the phantasms on the ground will build up damage. These will detonate if you resume casting or when they expire after 10 seconds.
Here’s a video showing how to build up damage. This was from the PTR before the Ring of Emptiness and Mundu 6 piece bonus change, but the Barber mechanics are still the same.