Tatas tad-vipakangunanam evahivyaktir vasananam. (IV-8)
Tatah = then; tad-vipaka = fruition; anugunanam = favourable; vasananam = tenedencies.
From these only those tendencies are manifested for which the conditions are favourable.
Life is a mix of good and bad. We induce good or bad tendencies (vasanas) in our life depending upon our good or bad karmas. Each karma (action), each thought leaves a residual impression or Vasana (a Sanskrit word which means the fragrance of food that is left in the cooking vessel even after it is washed) in our mind. Only yogis of high order, as we saw in previous sutra, who practice Niskama Karma, are free from the effects of their actions.
The Vasanas of our actions become tendencies of our personality including us to repeat the same action again and again and, thus, forming habits which become our typical characteristics. Impressions, the root cause of all the desires and resultant actions, accumulated over many birth from stock of our karmas, both good and bad (and mixed) called Sanchita Karma.
Vasanas are like the seeds that have to fructify or manifest (abhivyakti) according to the favourable circumstances (anugunanam) at appropriate time (vipaka). Our psyche is a very fertile land. Nature is very kind to allow each and every seed to sprout.
I may not have good looks but I 'wish' to be a model. This vasana HAS to fructify itself. Even if that requires a new body in some next birth.
This is how the cycle of rebirth goes on. We have to be careful in desiring something. All Vasanas have to manifest themselves according to opportunity present. These Vasanas are worked out in a particular birth from a given stock of karma (Prarabdha Karma) out of the total Sancita Karma.
Of course, a yogi knows the technique to come out of this vicious circle of Vasana-Karma-Vasana.