Yoga Sutras of Patanjali translated by Yogi Kalinath - Chapter, Verse 8.
Pada 4, Sutra 8
Tatas tad-vipaakaanugunaanaam evaabhivyaktir vaasanaanaam
From that (the three kinds of Karma), only the impressions where conditions become favorable manifest.
Through repeated actions we create impressions, grooves in our subconscious called Vasanas. These grooves are what are called memories ready to be reawakened, watered through repetition due to stimulation. In other words the right conditions becoming favorable for the manifestation or fruition of Karma.
Subconscious traits and reactionary habits are very difficult to overcome. We have to break the stimulus - response cycle of our outer (enticement of our sense organs) and inner (desires, memory and conditioning) environment. It takes much volition on the part of the individual to do that. Yoga is designed to create a gap between the stimulus - response paradigm in the consciousness of the Sadhak in order to instill control and hence make better conscious choices to alter one’s destiny.