And finally, Why's the other end of this string tied securely to your penis?

12 seconds sound clip from the Old School movie soundboard.

You can hear this line at 00:37:02 in the Blu-ray version of the movie.

Quote context

[...]

(Frank screams)

- Frank, just pace yourself.

- Copy that. Just got a little overexcited. Sorry.

- At this point you may be asking yourself, Why am I holding this 30-pound cinderblock in my hands?

- You might also ask yourself, Why does this cinderblock have a long piece of string tied to it?

- And finally, Why's the other end of this string tied securely to your penis?

- And the answer, ladies, is trust.

- This is your first test.

- Spanish! Do you trust we've provided you with enough slack so your block will land safely on the lawn?

- Sir, yes, sir.

[...]