How to build destructible signs on Minecraft?

I was playing my lava survival with a friend the other day and he recommend rather than destroying the wool blocks one-by one with TNT, I can use a sign to hold the lava and have it all instantly fall with a piston. But I never had much detail on how this would work?
Anyone have a description, video, or image?

Why not try holding the lava back with a normal stone and stick a sticky piston under it and then dig a underground tunnel and wire a redstone line and stick a switch to activate it