This effect is realized by "surface pressure."
As two belts catch and press the work, pressure is applied in the area, not one line or one point.
One example is curled films, Curles are often found as films are rolled on small rolls.
These curls become flat after surface pressure of the two belts.
4 films are curled as red lines tell.
Films are inserted. They are heated and press by the 2 belts.