I am not Yang Jun or Yang Zhenduo, so I cannot speak for them. Let me, however, summarize my own understanding of what I have learned by studying with them and reading Yang Zhenduo's book.
Yang Zhenduo advocates leaning forward slightly in a bow step whenever both hands are going in the same direction. That means all bow steps except single whip and fan through back and left ward off.
When you lean forward slightly and straighten the back leg (naturally straight, not locked) the entire body has the ability to 'prop' like a straight wooden pole, and any pressure exerted by an opponent against the front is then directed into the back foot and the ground. You can verify and tune the effectiveness of this by getting into the ding4 shi4 or end position of a move like 'push' from Grasp the Bird's Tail, and having a friend push against your two hands.
In a bow step, you must always have a slight resistance pushing backward by the front foot - in opposition to the push forward by the back foot. On this point see the article from Yang Zhenduo's book at http://www.yangfamilytaichi.com/info/essays/bow_steps.htm
(can also be found from the yangfamilytaichi.com site by navigating to info, essays, Bow Steps...). I have also heard Yang Zhenduo mention that the toes of the front foot on a bow step must curl downward and grab the floor in order to most effectively resist a pull from the front. You can also test the effectiveness of this with a partner.
Hope that helps.
[This message has been edited by JerryKarin (edited 03-08-2001).]
[This message has been edited by JerryKarin (edited 03-10-2001).]