To use HSS, you need a digital TTL flash that is supported by the V6 II (or the RF60).
With the YN560-III you should still be able to use PowerSync (aka HyperSync). That's an approach to use high shutter speeds that depends on the flash to be fired at full power. Hence, you lose the ability to adjust power and the exposure can be graduated but on the other hand, the light output is higher than in HSS mode.
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To use HSS, you need a digital TTL flash that is supported by the V6 II (or the RF60).
With the YN560-III you should still be able to use PowerSync (aka HyperSync). That's an approach to use high shutter speeds that depends on the flash to be fired at full power. Hence, you lose the ability to adjust power and the exposure can be graduated but on the other hand, the light output is higher than in HSS mode.