A woman from the host side will first call (karanga) to indicate to the visitors (manuhiri) to move forward on to the marae. See HERE for more details.
A woman from the manuhiri then returns the call as the manuhiri move forward onto the marae. The purpose of these two karanga is to weave a spiritual rope to allow the waka of the manuhiri to be pulled on to the marae. 

After the women doing the karanga have woven the rope, the haka p?whiri pulls on the canoe of the manuhiri, hence the reason 'T?ia mai te waka' is used at so many p?whiri.

The call of the haka powhiri likens the arrival of the group of visitors to the safe arrival of a canoe, with its paddlers and passengers, to the shore. The voices of the haka powhiri symbolically represent the rope by which the visitors are pulled safely onto the marae. See HERE for more details.

Often those doing the p?whiri hold greenery in their hands. The greenery should be specially chosen ensuring that there are both light and dark leaves, (often silver fern and kawakawa) representing life and death, and reminding us that that life and death are interwoven.

This theme of interwoven life and death is often reinforced by following T?ia Mai with the chant Ka mate, ka mate; kia ora, kia ora.

The use of  the Ka Mate' haka as a macho 'war chant' is a recent phenomenon.
This chant was used in the 14th century to haul ashore the migratory voyaging craft "Tainui." The crew thank their captain for navigating them into the warm life-giving sun. Aotearoa's icy cold sou'westerlies could be fatal for the lightly-clad voyagers from tropical Polynesia.