Complex sentence: This slows the exchange of energy between the primary and secondary coils, which allows the oscillating energy to stay in the secondary circuit longer before it returns back to the primary and begins dissipating in the spark.
Split into simpler sentences: This slows the exchange of energy between the coils, so that the energy alternates between primary and secondary slower. This allows the oscillating energy to stay in the secondary circuit longer before it returns back to the primary and begins dissipating in the spark.