As an example, a position that is encountered quite often in fast internet games occurs after 1.d4 d5 2.e4 dxe4 3.Nc3 Nf6 4.f3 and now 4..e5 (diagram).
Black realises he must develop quickly - he might have heard about playing the e5 quickly ( as in 1.d4 d5 2.e4 dxe4 3.Nc3 e5 ), but playing it on move 4 is just one move too late.
Play continues 5.dxe5 Qxd1+ 6.Kxd1 Nfd7 7.Nd5 Kd8 8.Bg5+ and by now, black should realise he is in a terrible mess. Black only has 8...f6, followed by 9.exf6 (diagram).
... a1/ 10...h6 11.Nxd7
......a1a/ 11...Kxd7 12.Bf6 (+-)
......a1b/ 11...hxg5 12.Nxf8 Rxf8 13.Kd2 (+-)
... a2/ 10...Be7 11.Nxe4 (+-)
... a3/ 10...Nc6 11.Nxe4 (+-)
b/ 9...Nxf6 10.Nxf6 h6 11.Nxe4+ hxg4 12.Nxg5 (+-)
So, that look's quite simple, doesn't it ? Just remember that black should not exchange queens on move 5 and play 5...Nfd7 instead ( white can follow up by 6.f4 and have a small lead ). Also, on move 6, black can improve with 6...Ng8 and white has only a small lead after 6...Ng8 7.Nd5 Kd8 8.fxe4.
Anyway, keep on winning these "won" Blackmar Diemer games !