This is a form that is usually found only in early Vedic texts. In Classical Sanskrit, this form is usually found after 'mA sma' (prohibitive) as in the example shown below, and is considered similar to the Imperative Mood. See [burrow2001] [1] and [KP2005] [2] for more details on this conjugation.
It looks similar to Imperfect and Aorist forms (and could be treated as one of these forms), but does not have the augment 'a' prefixed to the verb root.
Examples: