You need to be max jumping for at least 4 frames in order to rise a block, and have the possibility to double. When going full speed, this is impossible, as you either will hit the ceiling, or hit the wall too early. However, if you are moving between 11.8 and 13.2 speed, you can be inside the pipe area for 2 frames, which gives you enough time to get the double, and you have enough speed to get a double.

To perform the trick, you have to move between 5.6 and 5.9 pixels past the 3rd block (5.7 and 6.0 for the second jump), which you can gauge by having your left foot be right above the vertical line inside the block. Then, you hold flow, start running left one frame later, and exactly 9 frames into the run, release left and hold jump+crouch at the same time.