Quick FAE question for engineers that deal with drafting operations. I am a new firefighter engineer and I am having a little trouble understanding a concept relating to drafting. Heres how the drafting policy is set up at my fire department when we need to draft: The first in engine is fire attack. The 2nd in engine must get water supply. The 2nd in engine decides to set up a drafting operation. If the location is in a non hydrant area our box cards will already have tankers responding mutual aid. The second in engine will position itself in a good location for a drafting operation. The first in tanker will drop it's porta tank and feed the 2nd engine. The 2nd in engine will continue supplying the attack engine. The next tanker will dump into the porta tank, and it's at this time the 2nd engine will get a draft. 

Ok, so what i'm having trouble understanding is how the 2nd in engine get's a draft since he is already supplying the attack engine. I understand how to get a draft from nothing. Here our the ways that I know how to draft.

1. Make sure everything is closed and connections our tight. Throttle up to around 1,000 RPM make sure you are in volume mode and not pressure mode, open up your intake and pull your primer for 30-45 seconds and you should have a draft as long as everything is tight.

2. Another way is to obtain a draft is by using a low level strainer with a jet siphon. Connect a 1 3/4 line to the jet siphon and charge the jet siphon and this will pull water up the hard suction hose and you get a draft as long as your intake is open.

3. Another way to get a draft without using the primer/ if your primer is not working is to back flush the hard suction hose by Opening up your tank to pump/then open intake. Wait until bubbles go away then increase RPM.

4. Open up a high discharge like your deck gun. Then slowly open up your intake while increasing your RPM and you get a draft. 

So those are the 4 ways I know how to get a draft. How can I get a draft though if i'm already supplying another engine? If this was the case I cannot shut down the operation since there is another engine relying on my water. These other principles shouldn't work, right if i'm supplying another engine since everything will not be air tight? Also what if I give all my water to the attack engine before everything is set up, am I screwed because I don't have any water in my tank?