Spiritual combustion takes place when we step over the threshold of an obstacle in faith, and enter a new realm of the Spirit. It is like an explosion of fire in the natural when smoldering embers deprived of oxygen are suddenly exposed to increased oxygen, and accelerated combustion takes place.
I remember watching a video of a house burning. A firefighter was working through the house and opened a door behind which a fire was smoldering. When increased oxygen flow entered the room the fire exploded. Fire is a chemical reaction that requires fuel, heat, and oxygen. Those three components are called the Fire Triangle. Flammability increases as the level of oxygen increases.
Never underestimate the potential of a simple act of obedience. Your “yes” to the voice of God may have you to step across a spiritual threshold where a smoldering promise has been waiting for the wind of the Spirit to be released by your obedience. The fire God does not kill or injure when it accelerates in intensity. The purpose of God's fire is to burn away the lies and plans of hell that stood against the potential of a promise becoming a reality.
God wants to open a door in your life to allow the wind of His Spirit to go before you. The Spirit will ignite a promise that has been smoldering out of sight behind a closed door. He has been waiting for your “yes” so He can open the door. When your "yes" becomes an act of obedience aligning you with a promise from God, supernatural combustion will take place.