'Left hand' . A variety of explanations are given as a explanation but what in fact is this vamamarga is , as i think, a way back towards nature ( i.e gross).
The tamas ( Principle of inertia_ as ramakrishna defined the term) ,as i percieve, is the gross prakriti.
The rajas is lighter than tamas. ( the principle of activity).
The sattva ( the principle of goodness) is the lightest of all.
In fact the normal vidhya is of renunciation ( khadi vidhya..Ka ai ee l hreem .....), the process is like getting beyond these three modes of nature.
But as the tantra( web) means that there is nothing beyond these three modes of nature, all are thou that exists. Trying to get beyond nature is like destroying the self.
When a onion is peeled of layer after layer, in search of source of smell, we reach the empty inside. That means the smell was one with the onion. So, after realizing this fact the yogi turns back towards nature .......And this is called ' Vama marga'.

I tried my best, if im getting the concept wrong please discuss.