Luffy's ability to surprise and be unaware of what it was he was doing
has long been a staple of the series. His unleashing of the Conqueror's
Haki in the previous episode certainly shocked a lot of people on both
sides, even those that know of his bloodline to Dragon himself. Luffy
himself is completely unaware that he actually unleashed a power when he
screamed out like that as Ace was about to be beheaded and says as much
to Ivankov, which further cements just how frustrating Luffy can be to
others. His battle cry has however rallied the troops on both sides as
they won't take him less than serious and it has renewed the vigor of
both pirate and marine alike.
Whitebeard's approach to it all is rather interesting and shocking at the same time. He knows he can go only so far with what he's been through and with the navy men regrouping, it's going to be an even harder slog on him. So his chance to rally the troops around Luffy is an ideal solution since it gets them to focus on someone that's making active progress and is in pretty good shape overall. And after the Haki itself, they can rally easily around him and that starts them to overwhelm the navy men. The Marineford arc has seen a lot of back and forth, ebb and flow, in the battle and this is yet another one of them. Luffy at least has Ivankov along with him at this point who can add a little useful commentary to get Luffy to realize what's going on with the bigger picture.