"Before the starting whistle, the team takes the rugby field in loose formation. At first, they are silent and serious. Then they begin to chant and distort their faces, stretching their tongues and rolling their eyes. With hands chopping and slapping, the men shout. This isn't Hank Williams, Jr. asking if we're ready for some football. The New Zealand rugby team wants to know if we are ready to die."
It is a traditional Maori challenge/welcome. At the end they are supposed to place a spear or war club on the ground. If you accept the challenge you pick it up. Having a Haka performed for you should be regarded as an honour. Unfortunately, like many things these days, it is being misused and misinterpreted.