The cast is completed with a long, smooth haul on the forward stroke and a release of the fly line. The wedge shaped loop, unrolling in the air is a good sign of a successful cast.
You can cheat by “maintaining tension”. Basically that means you need enough sensitivity in your finger tips to feel the tension in the line. As you make your D-loop stroke with a haul and then start to feed, you judge the tension of the line in your fingers and ensure you are never left with a slack loop as in the photo above. If your line has enough speed to take the whole five feet of slack, you feed five feet. If the D-loop is only going slow enough to take two of those five feet of slack, you only feed two feet, and if there is no speed in your D-loop you just keep your rod hand and line hand far apart and tension between the line hand and the rod butt guide. This is “maintaining tension”. Of course, if your line hand does not slide back to your rod hand at the end of the D-loop stroke there will be nothing left for the forward haul.