The Friday after hours session is an illiquid session with even fewer participants than a normal after hours session. There is no S&P futures session. There is rarely ever any market moving news released late on a Friday and most institutions have packed it up. The best word to describe a Friday after hours session is random.
On Friday, I was debating whether to start building an SPY position at the market close but ultimately decided not to. The SPY closed at 107.39. I decided to stick a bid in for a starter position in the SPY at 107.21 in the after hours session. I believed that was a deep enough discount to the closing price to make the trade worth while. At some point during after hours my bid was hit. I had no idea the market would gap up this morning and it could have just as easily gapped down but with an 18 cent head start the odds were in my favor.