Okay so does that mean I can withdraw $300 worth of STEEM to poloniex in 6 hours?
No, but after 24 hours of your post you will be able to withdraw approximately half of your reward via SBD.
In the OP you said you got paid $390 for a post, so you will get approximately 50% of that which is $195 USD worth of SBD. You can withdrawal that immediately and sell on Polo.
The rest is paid in SP which is locked in until you "power it down". After it is powered down, it is paid in 104 equal weekly payments over the course of 2 years.