There's nothing mysterious or even fake about this. I've done this either with one student or a line of students.
In Tai Chi, there is a concept called the "ground path." Most martial arts use it, but tai chi uses it more -- the problem with tai chi teachers is that most of them are poorly trained and focus on silly stuff like "chi" instead of the proper body mechanics.
When a boxer punches, the power starts with the ground, goes through the leg, hips, arm and fist. Same is true with Tai Chi, which is actually a very powerful martial art when you do it right. Tai Chi is more relaxed, however, and sets up this ground path through body structure and alignment.
So, you can get a big guy to push on your arm and you can remain relaxed, and he isn't going to push you over if you're properly grounded. You can get several people to line up behind him and push, but if you're grounded, you'll be able to hold it back.
I've studied with the folks who created tai chi -- the Chen family, and they can break someone in half before you can blink an eye. Unfortunately, most tai chi is Yang style, and they've attracted a lot of nuts who believe in the mystical. As a result, their tai chi isn't good and they can't fight at all.
There are free sample videos on tai chi fighting applications you can sign up and get by email on my website -- www.internalfightingarts.com