I’ve instructed the congregation at different times. Sometimes before we sing, sometimes during the singing, and sometimes during a message. Equipping people to understand what you’re doing while you’re singing is both an act of worship (it’s building up the church), and a means of worship (it helps people participate with faith). It may interrupt the musical flow, but that’s not the same as interrupting the Spirit’s work in our hearts. If what I say is clear and engages people’s attention, usually the flow continues. When I “feel” as though I’m interrupting the flow, I usually end up being timid or confusing.