Why there are three convolvers in "Two"StageFFTConvolver class?

[menzi11]

Something in "TwoStageFFTConvolver.h" really confusing me.
When I saw "TwoStage" I think it is easy to understand that "TwoStage" means a head conv runs in audio thread, and a long tail conv runs in background thread. Those two conv makes a convolver realtime, but when I look inside the code of TwoStageFFTConvolver class, it's more like a "ThreeStageFFTConvolver".

My point is, why there are three convolvers? A head, a "tailConvolver0" and a "tailConvolver"? I know "tailConvolver" runs in background thread, but what "tailConvolver0" is doing here? It runs in audio thread and has same block size with head conv. I really don't understand why we need two conv in audio thread? can't one head conv can do the job?

Appreciate for your kindly help.