how does PeriodicExportingMetricReader 's exportTrigger get signaled

[slowjams]

opentelemetry-dotnet/src/OpenTelemetry/Metrics/Reader/PeriodicExportingMetricReader.cs

Line 111 in 3f86f47

var triggers = new WaitHandle[] { this.exportTrigger, this.shutdownTrigger };

I can only see this.exportTrigger.Dispose() called in the Dispose method, but there is already a shutdownTrigger which matches the purpose of the Dispose method, so why exportTrigger is being used when it can never be signaled other than shutdown?