Hi Guys -
Before I wrote for a living - I owned a hobby store. Before I owned a hobby store - I was national sales manager for 2 different high pressure washer manufacturers - and was in charge of product design and development for a third firm. So - I feel like I can answer the question of "how does this work".
The injector contains a venturi - the passing of the water through this venturi creates a low pressure area in the siphon chamber. The sand is then drawn into this chamber, where it mixes with the high-pressure water, it then exits - typically through a ceramic discharge cone. Washers suitable to use with sandblast attachments typically are in the 4 to 6 gallon per minute - 2000 to 3200 psi range. Thus, the sand is impacting the work surface under MUCH more pressure than is the case with a pneumatic sand blaster....hence, with experience, you can really work quickly.
Because of the orfice, the sandblast attachment should be sized to the specs of the washer it is used with - this is NOT a one-size fits all item. The orfice is a wear item - so pick up an extra one when you purchase the unit, as well as 2-3 of the discharge cones. The wear on both of these parts is gradual, so you won't notice a degredation in performance - but trust me, performance does decline with time.
Ospho would be a great additive to prevent flash rusting.
Hope this helps,
David Doyle