Hello,
When you empty the trash, the files are still physically there. But, the space they occupied is marked as available for use. So, they are not taking-up any space. That space is immediately freed, and made available for use by the next program or file that needs it.
But, because it is stored magnetically, that file can be retrieved by manually (or with a special program) going back and locating where that file was stored and re-assigning a name and a pointer to it.
The only difference between regular empty trash and secure-empty trash, is that the secure option will immediately over-write that deleted file's space with garbage a couple of times to make it more difficult for someone to recover (or undelete).
That is the only difference. Otherwise, the space is still storing magnetic information regardless of which method you use. But, it is not wasting space either way. The space is marked as free, and will be used as needed.
I hope this helps.
Let us know if you have other questions.
P.S., if you'd like, go ahead and click the "Helpful" or "Solved" buttons on any of the posts / replies above if you feel they were helpful or adequately answered your question.