YouTube: http://www.youtube.com/watch?v=_7a9GzrBFGE

However, the performer has slowed down the notes in the 2/4 measures to twice their usual duration, possibly because the tune was misnotated in his British tunebook. Compare with the following clip, which begins with the same tune in its proper timing:

Stadtkapelle Schongau - Zwiefacher a la Blechschaden
http://www.youtube.com/watch?v=bbmZhsdOnT8

And the same Zwiefacher being sung:
Unsre oide Kat: http://www.youtube.com/watch?v=W2TZ8Ti8Ma8
Die alte Kath: http://www.youtube.com/watch?v=R-TPJ8HHNf4

Zwiefacher switch between 3/4 (waltz) and 2/4 (pivot) measures according to a pattern particular to each piece--you have to know the tune and its pattern before you begin to dance. The oom-pah or oom-pah-pah nature of the accompaniment is your best guide.

Here's a set of a Zwiefacher clips demonstrating the customary way of switching meters and dancing to them; you can see the interleaving of waltz and fast pivot steps, and how the pattern varies from one dance to the next:
http://www.youtube.com/user/zeifacher
(Yes, the channel's name is "zeifacher" without the W.)