This is a clip from Undead, a 2003 Australian zombie-science fiction-comedy-horror film and the debut of filmmaking brothers Michael and Peter Spierig.
In the film, a small town is hit by meteorites that turn its residents into ravenous zombies. Only a small group of people remain unharmed and must find a way out.
This clip shows a common filmmaking technique, with the subversion of the climax by a moment of anticlimax, only then to switch again to climax. It is an old movie trick, often employed for comic effect, and it is used effectively here.
Apart from the obvious humour of the unexpected, this short clip is tightly edited (by the Spierig Brothers) and the use of suspenseful music (by Cliff Bradley) helps to build tension.
Undead starred Felicity Mason and the innovative, self-financed and wildly entertaining debut became a calling card for the Spierig Brothers. Their follow-up film, Daybreakers (2009), starred Ethan Hawke and Sam Neill and received a wide release in US cinemas, launching their Hollywood career.