Waveform

The Twelfth Doctor described audio waveforms as looking like landscapes of mountains and valleys. When a waveform was flipped, for instance by inverting the phase of a recording, its "valleys" were replaced with "mountains", and vice versa. When a waveform met its inverse, they would both be cancelled out. (AUDIO: Dead Media)

Waveforms could pass through physical barriers. Hence, when Vardans took the form of waveforms, they were able to do the same. The Vardans could take the form of waveforms, and hence pass through physical barriers. (AUDIO: Wave of Destruction)

According to the First Doctor, points in history became fixed once a probability waveform had collapsed. (AUDIO: Daybreak, The Vardan Invasion of Mirth) It was dangerous at this point to interfere. (AUDIO: Daybreak) The Doctor intimated that the Vardans ought to recognise a probability waveform, given their nature. (AUDIO: The Vardan Invasion of Mirth)