Union of what? Yoga is the union of will with power.Will without power, cannot get things done. Power on its own is just chaos, if there is nothing to guide or harness it.
Yoga stands true to its name in every sense of the word. If we look at its operational aspect, whether it is hatha-yoga, raja-yoga, siddha-yoga or any other school of yoga, a yogi aspires to awaken vast measures of power (Kundalini shakti), lying dormant in his root chakra. Then strives to propel this power upwards, to the chakra that is the seat of the ‘will’. Thus uniting will with power.
The union of Shiva with Shakti. The union of Yin and Yang.
Merging of all dualities into a singularity.
Even cause and effect do not remain as two events separated by time. Similarly thought and action become one. When thought and action are the same, time dissolves and everything happens in a trice.
Perhaps the biblical verse “Let thy will be done”, makes more sense now.