By pressing a rice bowl on his abdominal muscles, Zhang Xingquan was able to create enough suction to pull a 36.15 tonnes heavy train for 40 meters
Source: Guinness world records website.
How does the bowl 'attach' to his abdominals and not fall down as he walks backwards? How is he able to use suction to pull such a heavy object?
A video can be found here.
The exact mechanism you describe for how suction cups work is how the rice bowl work. Instead of the bowl being flexible, though, it's his body (skin and muscles) that are providing the change in volume necessary for the suction. So, instead of the suction cup creating the volume change, it's the surface the suction cup (the bowl) is attached to, Mr. Zhang's very impressive abs.
All you need for suction is for the air pressure inside the bowl to be less than the air outside the bowl. I presume that Mr. Zhang accomplishes this by filling as much of the bowl as he can with his own skin and muscle, and then uses impressive abdominal strength to pull that skin and muscle back, increasing the volume accessible to the air in the bowl and thus decreasing its pressure.
Also note that Mr. Zhang has a strap that helps hold the bowl to his gut. I am not implying that this strap is being used to actually hold the bowl onto his gut, but probably more to keep the bowl positioned well enough so that the suction is not broken.
We must all keep in mind that for average atmospheric pressure, and assuming Zhang could pull a hard vacuum with his abdomen (which is probably not feasible, but serves to provide us with a bound), the maximum (negative) pressure he could achieve is only about 14.7 psia.
Given 36 tonnes, you can back calculate what the diameter of the bowl would have to be. It turns out to be about 80 inches in diameter. And that doesn't even take into account any frictional forces that have to be overcome.
So Zhang either has a tremendously large abdomen (and a large rice bowl to match!) or something is amiss in the record reporting.
