Final answer:
The trapeze artist in the passage swung from the second trapeze by his legs immediately after releasing his grip from the first trapeze.
Step-by-step explanation:
The trapeze artist in the passage released his grip from the first trapeze and immediately swung from the second trapeze by his legs. This is shown in the sentence, 'He narrowly caught a second trapeze bar and swung from it by his legs.' After releasing his grip, the trapeze artist performed a triple flip and then caught the second trapeze to continue his act.