Going back to starting turbines …
Conceptually they all start the same way; the devil is in the details.
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Using an outside force, spin the innards of the engine up to a low-ish RPM, maybe 20% of max. That gets enough airflow and compression going through the engine that a fire, once lit, will be helpful not hurtful.
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Activate the ignition, and add a low flow of fuel. The fire should start promptly. Given the starter- and inertia-driven flow through the engine, the fire will want to flow out the back, not the front, and will itself begin to contribute energy to increasing the RPM.
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Watch everything for awhile while the engine slowly climbs up from the equivalent of a car “lugging” up to an RPM where the fire alone is producing enough output to keep the engine accelerating. Along the way the rate of fuel flow may need to be adjusted.
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Once that self-sustaining point is reached, cut off the power to the starter which disengages mechanically from the engine rotor(s). The engine / fire is now self-sustaining, but is still well short of idle. Watch while it finishes bootstrapping itself up to normal idle and the temperatures, RPMs, oils pressure, & temperature etc., settle down to normal idle values. Something around 40% of max RPM is normal idle.
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Apply the external loads such as electrical generators, hydraulic pumps, and air bleeds that were left off to keep the engine as unloaded as possible while trying to start.
In primitive engines, all the sequencing of those things and all the monitoring for normal vs abnormal progress is 100% manual. In modern fancy engines, it’s “push the button & let HAL handle the details”.
Even in modern engies, the start is walking a bit of a tightrope, but at least with nice safety nets below. In primitive engines, the bootstrap regime is a very narrow ledge along a precipice with nothing but your wits to save you if you fall.
If too much fuel gets in before the fire starts, the fire overwhelms the airflow and you melt the engine. If while it’s accelerating under starter assist, you add too much fuel too quickly, the fire overwhelms the airflow and you melt the engine. If the starter falters mid-start, or your finger slips off the switch, the fire overwhelms the airflow and you melt the engine. If there’s too much tailwind while you try to start, the extra back pressure may make the fire overwhelm the airflow and you melt the engine. If the engine is old, or dirty inside, or the starter is weak, or the …, the whole bootstrap is even more critical. If at any point the fire overwhelms the airflow you melt the engine.
Finally, if you do need to abort the start as things are trending towards haywire, but not yet there, there’s one right way and a dozen wrong ways. The right way may cost you an inspection of the engine at worst. Any one of the many wrong ways may melt the engine or even set the aircraft on fire.
Perhaps you’re sensing a pattern by now. 
On an old turbine helo or airplane like that heli-skier, the engine may be 80% of the value of the whole aircraft. Or more accurately, the remaining expected lifetime of the engine is 80% of the economic value of the aircraft. When the engine is done, the whole aircraft goes to the boneyard forever, net of a few scavenged parts. There’s a narrow economic ledge of using up the remaining life of the aircraft at enough profit per hour to afford to buy the next mostly run-out bucket-of-bolts and consume the last of its economic value in turn. Burning up the engine prematurely knocks the business off that narrow economic ledge, perhaps fatally.