What's the "trigger" for Extraordinary growth of runner beans from tubers? in Barbs garden.

Runner beans produce edible "tubers" that often survive the winter in Ireland, the UK and in Canada. Then they explode into growth in the spring! But usually way too late to produce a decent crop. What is the "trigger" that gets them growing again. If we can trigger them reliably, they could produce a super early crop, way before beans from seeds!