I was happy with the explanation when I looked at this in the end.
Basically if you go from a contract when you were had oil for $18 a barrel (buy) when the contract closed, and the new futures contract is $25 a barrel (buy) you will be deducted $7 for every barrel you own. So if you own 100 barrels you’ll be deducted $700 from your account, however, the barrels you own are worth $700 more so it equals out.
HOWEVER - what I have worked out is that is does change your margin position. The “blocked funds/margin” on the future at $18 are less than those on the future at $25. So if you were close to 25% margin called then the rollover could drop you below the 25% where positions are automatically sold. This is the danger with a rollover where the difference in prices between the 2 futures contracts are big. (ie oil currently ~$18 to $25). I think the rollover will take my position from 56% margin to 54% so it has an effect.
I’ve created myself a simple spreadsheet in excel so I can understand exactly what the effect of certain price changes, rollovers, future buys and sells will be so I can ensure I enough funds to cover certain scenarios ie oil dropping to $5 a barrel.
Hope this helps. I don’t work for 212 this is just what I’ve worked out.