you take the white off the glass jar. pour your paint in the jar. put the can back on jar. spray paint your project. clean up spray can. inside the can is pressurized aerosol. when you spray it creates a vacuum in the tube and sux the paint out of the jar.
instead of having a left over full spray can from a project where i needed 2 oz's, i got that little aerosol can. when i got my guage pod, i went to the auto body shop, had them mix me 3 oz's of dye, and left. cost me about $3, as i already had a tin can for the paint. or you can buy a whole spray can of dye for $20+, and let is sit on the shelf to probably never get used again.
that thing is awesome. go to the paint shop, color match ANYTHING, and you can spray it on, using only what you need. i mean, how many extra rattle cans do you have laying around from past projects? or how many have you thrown away for bein old, or didnt spray worth a damn any more, or lost the aerosol on the inside altogether.
obviously its not for larger projects. patio furniture for instance. but to touch up that furniture, or smaller projects, it rules. i did my guage pod, my grill lid for my gas grill, and touched up some headers for my buddies bbc, all about 20 mins apart from each other, with that little spray can.
helps i got a real good paint shop also.