push pull swing

[RUMI]

I have recently gone to an amusement park where i found a strange swing..
 The strange thing about it was that its amplitute can be varied by the persons sitting on it..(by pushing the rod near the feet ) .. My doubt is ''how can an internal force affect the system??'' Or is something else happening there...??

Please refer to the fig. 1a in the second page in http://www.google.com/patents?id=w7BnAAAAEBAJ&pg=PA2&source=gbs_selected_pages&cad=2#v=onepage&q&f=false

When the swing is moving down from left, if the person on left pushes the foot rod, the subsequent amplitude gets increased...
 
Can i know the physics of this ??