[spaghetti]

Aspetto means "I'm waiting". The technique was given this name in the early 50's by a group of spearfishing pioneers in Tuscany (at Marina di Pisa) led by italian legend Rodolfo "Marò" Betti, who is also saidn to be the inventor of the speargun reel. He passed away late last year. Rest in peace.
As Bgill said it's basically about lying motionless on the bottom trying to "blend-in" with a rock or weeds, or just the bottom itself and waiting for unsuspecting or curious fish to come into range.
The fundamental is choosing the right place, both in shallow or deeper water. The ideal is having current and sunlight from the back, which is not always possible. You have to be hidden, but not completely: the fish must see "something" to catch their attention and turn on their curiosity. it's okay to make your head partly visibile, and your gun partly visible too.
About choosing the right place, the same considerations made about Torbido apply to aspetto as well: rocks, bushes of kelp, "steps", wreckage, everything you can hide behind. Moreover, these things attract fish. You can use sounds or other tricks to attract fish: a deep "gu-gu" from your throat, a clack clack with your teeth (sounds like fish cracking mussles), or release a bubble of air from your mouth (not from the snorkel).
One tip is to watch the bait, the groups of small fish. If they stay compact in balls and move nervous, there will probably be a bigger predator around. Point your gun towards the bait and see what happens.
Once that the fish comes to you, for curiosity or for remarking his territoriality, shoot it soon as it is in range. Don't wait for the fish to turn showing his side: if it turns, it means that he lost interest about you, or that it's scared, so that in both cases he's running away. Shoot before he turns, aim frontally, straight in the face.
The proper gear depends on visibility and type of fish targeted. The gun must be at least a 75 if the viz is poor. Take care of a correct weight set: weights will keep you motionless on the bottom, which is very important. Ankle strapped weights, to prevent your fins from waving like a flag, help a lot for Aspetto.
The dive itself must be done with care: don't dive directly on the area where fish are supposed to be. Go down from a reasonable distance, don't paddle with the fins, just let your ballast push you down. When you're near the bottom, crawl to your hiding place without finning, use your free hand to help the crawling.
If you dive deep, do the classic inverted "L" dive, while in shallow you'd better go down obliquely in order not to show your whole shape.
Never push your breath hold capability to the limit: always leave some spare air in case of emergency (tangling for example).