I'm not sure if this is defined as wanking, but I like to think that Broly's chi is mostly ambient, but with his signiture. Ignoring some climactic dialogue, I'd say that this ambient chi is also what kills him.
This, for me, solves a few things.
1. The characters donating power to Goku at the end. I'd like to think that instead of gathering internal reserves, they gathered the ambient, green chi and pour into into Goku.
2. Broly's ki blasts and bubble-shield, which more memorably gathers from the surrounding area, rather than from his cupped hand.
3. Broly's size and lack of pupils/irises, which might actually act as facilitators for managing this chi and gives up sight so to better attune Broly to it and the objects that enter his field.
4. His transformation sequence ripping him apart from the inside out. From the start, it probably threw out all the energy from the shift, which would explain the fancy color changes.
5. Going back to #1, his end-of-movie explosion.
6. Earlier in the movie, Goku finding Broly's chi residue.
Broly Getting Stronger
Re: Broly Getting Stronger
That which is, is bound to be
That which is not, may never be
In this sense, no things are free
Wether they may or may not be
That which is not, may never be
In this sense, no things are free
Wether they may or may not be