The first problem I see here is that- muscles only contract. So every motion is really a 'pulling up' because it's a contraction. The muscles can't 'push' anything, they all contract, and make the body do different things because of their orientation in the body. So triceps seem to be making the arm 'push out' but really they are pulling on the backside of the arm to make the arm extend. Muscles work in opposing sets like this so we can get action on both sides of the body. We both are fully aware of this I'm sure, but I have to say this to make my next point.
If you use one set of muscles to support or lift a load, they are pulling. you can use an opposing set of muscles to pull in the opposite direction at the same time ('dynamic tension'). But you don't get a bonus result for this, you only expend more energy. Muscles can't help each other by using a complimentary 'push and pull' or 'extend and contract', they can only contract or 'pull'. So if you activate opposing muscle groups you are only costing energy and not gaining it.
sure the muscle contracted and pull. however, muscle isn't the only thing in this model. don't forget the bones and sinew and other stuffs which create various levers and pulleys. question for you, do you use the same set of muscles to pull and push? or different set of muscles? another question for you, how do you generate a push if you only have muscle pull?
i think we posted this one before on the study about muscle usage using Kuroda sensei as the subject.
Look at the graph comparing between normal folks and Kuroda sensei. you would see that Kuroda sensei used his muscle differently than regular folks. then look at the graphs again and think in term of efficiency and energy expenditure. now replace Kuroda sensei's sword with a small weight and you got the exercise from Sigman that i mentioned. essentiall, "get underneath" the sword when you lift, and "get on top" when you cut.
Now this is not to say that the 'imagery' of pushing with one set of muscles while pulling with another set isn't helpful. But that can't be what is actually happening. Imagery is very useful in training physical motion. I often use images of rushing water, or electricity going through the body to help my students move correctly. But of course we know there is no water or electricity rushing through the body.
in order to do these stuffs, we have to use imagery to mass manage the actions of our body. it's almost impossible to control the firing of so many different muscle groups throughout your body at the sametime or in multiple sequences. imagery allows our brain to do that without us interfere with the process. i don't use rushing water imagery, not when i am wearing the full aikido gear and can't get the damn gi pants off fast enough, especially after i down a whole pitcher of tea.