...it is all Giku's own force. But Hakuho crafted this masterfully (well he is The master after all). This technique is commonly used in Mongolian wrestling. The wrestlers are opposing each other with lots of force, but at the same time watching, or feeling each other's move so carefully that any small movement will start their next round of attacks. It is extremely heightened situation. In a fleeting moment Hakuho makes such a slight movement as if he is beginning a big push. Giku's reflex responds even though his mind probably knew what Hakuho is up to. Giku's body stiffens and braces for Hakuho's push in a fleeting moment almost involuntarily by reflex. Now Giku lost his frontal footing, positioning his body in extreme angle, and his other leg is so far behind to counter Hakuho's push. It is time for Hakuho to tap that back leg even further while pulling Giku's upper body forward and down.
It is timing. If Hakuho missed Giku's reflex response then this would have been ineffective. Hakuho's preparation for this move is so subtle that we may not be able to see it from TV, but Giku probably felt stiffening of Hakuho's body in tenths of a second before realizing what it was.
In a way, Hakuho controlled Giku's reflex, true show of a master.