The structure of your example is:
[S But then he took a great leap, [S (he was)
trying to pull a high kick out of the sky ] ]
or, more schematically, [S ... [S ... ] ] , that is, it is a sentence (S) within a sentence. That is what a complex sentence is: a sentence with another sentence inside it. I've added the "(he was)", which I take to be understood elements, to make sense of it.
The outermost sentence is the independent part and the inner sentence (with the understood elements) is the dependent part. In this particular case, you can tell the independent S by the fact that it is the one with the finite verb "took". Finite means tense-bearing -- "took" is in the past tense. There is no explicit finite verb in the dependent sentence, but the understood "was" is a finite verb.
Looking for a finite verb is one strategy for identifying independent clauses, and it works here, but it won't work in general. I can't think of a single rule that would always work to identify independent sentences.