Everything that you trade on the exchange is a future and not a forward. Basically when you trade during the compulsory delivery period even though you may not intend to take delivery or give delivery, if any guy with an opposite position intends to, you can get assigned.
So basically, assume you are long 1 big gold and have 1.3lks in your account and you carry this forward to the compulsory delivery. If a person who is short gold intends to deliver, there is a chance that you are assigned.
Once assigned, the margin increase is not incremental but complete. So if gold is at 30000, you will need to transfer 30lks to your trading account immediately. Once the money is in your trading account we as brokers will have to take delivery of 1lk physical gold from a warehouse, typically for gold it is in Ahmedabad. We will then need you to pick this 1kg gold brick from us.
As you would realize this is a pretty cumbersome procedure. I have had a friend who had to forcibly take 10 quintals of turmeric
because turmeric was hitting circuits and he could not exit before hitting compulsory delivery.
Imagine 10 quintals of turmeric..
..
no online broker today lets you hold a position to compulsory delivery period, you could try some of the offline ones..