Combine each pair of sentences below so that one sentence is added to the beginning of the other in order to state a fact or answer a question.
Who watches the ventriloquist carefully? They will discover the secret.
From your question, I think what you are after is a way of combining the two sentences that would answer the question of how is the ventriloquist giving voice to the dummy that is in his hand. Looking at the sentences, we can easily do this if we make them into one statement and placing the first sentence towards the beginning of the new structure. Therefore, if we apply this, we will come up with the following statement:
To discover the secret of how the ventriloquist manages to throw his voice, you need to watch the ventriloquist very carefully.
This melds the two sentences together, creating one sole sentence that is used to state a fact but also to answer the implied question regarding the ventriloquist's skill in throwing his voice.