M1 vs YO
A YO is simply you bringing the yarn under and over the needle (and sometimes under again if your next stitch is a purl st), which as previously stated is used to gave a lacier effect or holes to pass ribbon or string through. M1 requires you to pick up a bar from between the two needles either front to back or back to front and work through either the front or back of the stitch. It gives a smoother fabric with a less noticeable hole. The two methods are generally not interchangeable. Check out the videos than you can decide what works best with your project.
Judge: Madame, either you will have to cease knitting or I will have to cease judging. Which shall it be?
Miss Marple: It helps me to concentrate, milord.
Judge: It does not help me, madame.
Miss Marple: Oh. Very well. (puts away knitting)
Judge: Thank you, Madame.
The lyf so short, the craft so long to lerne.
~ Geoffrey Chaucer - The Parliament of Fowls Line 1.