>>4900343>>4900491here
the air is just pushed out from under it because the gas has a greater force than the resisting air.
but the combustion chamber has a greater force than the gas, so pressure builds up in the engine because it's being created faster than it can escape downward (green arrow).
conservation of momentum = that built up energy has to go somewhere, and since it was trying to go down, third law = opposite reaction transfers the energy upwards
