Smart Sim real world use?

I read on the honestmobile site that the smart sim locks in when the primary sim signal drops out.

The question I have is that I often find on my commuting route that my mobile signal drops to a single bar, but data flow seems to cease despite this minimal connection. In this circumstance, I presume my phone will not automatically switch to the smart Sim. Is my understanding correct? Or can I manually switch over?