The DVD repeatedly emphasises the key point (for Declarer) that ruffing in the short trump hand gains tricks, whilst ruffing in the long trump hand does not.
(There are good reasons to ruff in the long trump hand, such as to establish a long suit, but the ruffing in itself does not increase the trick count.)
The second part of the DVD goes on to look at things from the defenders' point of view. Recognising a short trump holding and shortage elsewhere, defenders should assume declarer will attempt to ruff, and therefore defenders lead trumps at every opportunity.
Essential viewing for all players