Help us out here with a little bit more context.
First of all, thanks for taking the time to publish such practical and informative material on weight training (and answer dumb questions such as the following ).
In "Strong Enough?" I don't quite understand the comment: "...the limiting factor is the ability of the trunk to support the weight while it is lifted, and that doesn't get trained on the bench."
Why isn't the limiting factor the strength of the triceps or some small contributing muscle group? Also, how is it not trained - if it's a contributing (and limiting) factor?
Help us out here with a little bit more context.
Page 8, second paragraph. I think I get it now though - the limiting factor in upper body strength (as developed by the press for instance) is the trunk stabilizing that weight. So you're not developing the weak link in upper body strength, making bench press strength less transferable to other upper body exercises (e.g. pressing).