Can You Whistle Really Loudly Using Your Fingers?

Yes.

Insert both index and middle fingers in each side of your mouth, two to a side. Fold back about 3/8" of the tip of your tongue over the top of the rest of your tongue and press your lower lip backwards over your lower front teeth, using said fingers to hold it down. The tips of both middle fingers should touch forming a "v" inside your mouth.
Blow real hard.

This is a very useful technique for getting famous rock stars to come back on stage for their mandatory encore number. Especially since matches and Bic lighters were outlawed by a Constitutional Amendment and have been replaced by cell phone flashlight apps.

I believe this what sent Grace Slick into retirement and reduced her to painting portraits of White Rabbits all day long.

I have used this technique many many times at all the concerts that I can remember going to, being at, or was informed that yes, I was there too. Although it does not work real well if you can't get up off the floor.

It is also handy for annoying concert goers who can't do it as they get very envious of your talent and glare at you when they notice how much the rock group up there on the stage appreciates the gesture.

A word of caution. Hyperventilation may set in if you persist in whistling for over 15 continuous minutes trying to get Bob Dylan back on stage after he walks off without an encore and you paid a hundred bucks to see him...what an asshole.

Also that time David Crosby refused to sing cause the crowd was too loud and Nash just stood there and said, "Well that's David." Whistling didn't help...what an asshole.

Another word of caution, some concerts goers seem to dislike having you whistle at 10,000 decibels in their right, or for that matter left ear, for some reason. If you persist a black eye may result...assholes seem to be everywhere at concerts lately.

I will be gracing Carlos Santana with my gift soon.

 

That's the idea, you have to make a venturi to get a narrow air passage. As the passage narrows the flow of air speeds up and vibrates due to turbulence. This creates a ring vortex which is projected backwards into your mouth which then becomes a resonant cavity. The backward flowing vortexes bounce of the back of the cavity and then travel to the front, as they travel they accumulate and resonate at the same frequency. As each vortex gets added to the next vortex the sound increases, it gets louder. If you move your fingers around you change the shape of the cavity and thus the vortex waves take a longer or shorter time to travel across that cavity and this either raises or lowers the pitch.

Some people can do this with only two fingers, or by folding their lip and tongue a certain way. I need all four fingers.