The basic principle is that you compress air into the tank (using the donkry engine/compressor) then when you get the tank upto pressure, you can release the product.
With a non-tip tank like this there are compartments called pots. This one has three and you have to empty each one, one by one. This is done by opening a valve and with the pressure in the tank the product comes out. Also at the same time there is another valve from the compressor which enables you to blow air up the line to which the product is comming out. This helps get product up the line into the silo.
That is the very basic principle, however there is alot to learn with pressure tankers. They can be very dangerous and cause alot of damage when things go wrong. Getting the product blocked, and having to blow it back. Blowing the lids on silos is a common one. Also blowing the product all over the place because the lines are not connected correctly.
Best to have all the training and spend two/three weeks with an old hand to learn the ropes.
Hope that helps abit
Phil