Throttling (arbitrary frequency limitation of the CPU) happens when a sensor is reading bad, a bad battery is used or battery is absent. The machine becomes slow but not always totally unusable. Youtube video playback can become choppy at higher resolutions, moving cursor halts video, etc... The CPU temperature won't go up in most cases (and the fan will either stay quite low or ramp up fully).
The following example is a 820-00165 (MBA 13" 2017 i7) with prime95 torture test starting in the middle of the graph.
Notice the frequency capped around 1GHz, the low power used and subsequent low thermals (under 65°C).
The frequency now scales up and stays there (normal thermal throttling) as temps rise up to 100°C (Prime95 small FFT is torturing the poor Air).