The earth-terma are physical objects — which may be either an actual text, or physical objects that trigger a recollection of the teaching. The mind-terma are constituted by space and are placed via guru-transmission, or realizations achieved in meditation which connect the practitioner directly with the essential content of the teaching in one simultaneous experience. Once this has occurred, the tertön holds the complete teaching in mind and is required by convention to transcribe the terma twice from memory (if of textual nature) in one uninterrupted session. The transcriptions are then compared, and if no discrepancy or inconsistency is evident the terma is sealed as authentic.
Source: Terma (religion) – Wikipedia