It's nothing to do with heat it's purely mechanical - just as it was with modified brushed motors.
If you throttle up off load, there is significantly less current draw on the motor. However, it is able to spool up significantly faster also. With a "dynamic timing" ESC, the timing is generally increased as the rpm rises. Because the RPM rises off load quicker than it would on load, it adds more (and more) timing.
If there is no load on the motor, it is able to achieve significantly higher RPM than it would when loaded. The rotor can then mechanically explode - which creates a big mess!!
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Matthew White
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