Block Hit in Minecraft?

I have often heard on Youtube videos that you should blockhit PVP in Minecraft, can I possibly explain to me how to do that?

Only goes in 1.8, Actually you just press both mouse buttons at the same time when you hit. This way you can get on better players with less cps

Thank you

This is only possible in 1.7 and 1.8 with labymod. You have to press both mouse buttons at the same time.