It seems very similar to Trotsky's transitional program. However the resources I've found online explaining it have been few and short, so I'm still confused how mass line actually works in practice. It seems like a fantastic tactic for the left to deploy right now, but I haven't found any resources that explain it in detail.
You should read this pamphlet: Some Points on the Mass Line .
The principles it outlined were very helpfull for me when I was organising protests.