My understanding of the transition from hydrogen shell burning to helium flash point:
The hydrogen shell burning occurs because the hydrogen in the suns core during the main sequence has been used up, gravity wins at the core and it contracts. Temperature goes up and there is a region around the core where the temperature is large enough to start fusion in the hydrogen in the shell. The core continues to contract because the increase in pressure and resistance to gravity from the shell isnt large enough to stop the contraction. The helium begins to fuse. There is a big flash because the helium fuses uncontrollably.
Why does the helium fuse uncontrollably? Also as the core contracts, shouldn't larger and larger regions around the core start to fuse hydrogen? I.e shouldn't the shell actually increase in size as the sun goes through the hydrogen shell burning phase.