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 stay quite low).
The following example is a 820-00165 (MBA 13" 2017 i7) with prime95 torture test starting in the middle of the graph.