If you want to highlight two or more techniques or forms on a video, just tap the microphone button at the top of the screen. If you want to record your voice over the video and make drawings on the video, select the first option, "Record Screen & Microphone."
As the video is being recorded, you can use the scrubber wheel at the bottom to go forward and backward. You can draw on the video while it records your voice. When you want to clear your old drawing and make a new one, tap the "Clear" button near the toolbox. You can pause your recording while you do this and resume recording once your edits are made or just record right through.