Where does the strength come from?

Qigong, neigong, form, connection, alignment and gravity.
Until you believe in the neigong and have faith that it exists within your every movement, you remain tense.
You imbue your form and applications with muscle tension in the hope of possessing a strength that is already present.
The irony is that you cannot use your whole-body strength until you stop exerting externally.

External effort is about trying, about doing - whereas the internal is about allowing.
If you have trained neigong for months and practiced qigong regularly, your limbs are already strong; so doing is not required.