If you are still all fog up as to what is a MOW tip, George Cook gives a great concise introduction on MOW tip in this video.
Instead of the traditional sink tips, MOW tips are increasingly used as weapon of choice for the Skagit guys targeting Steelhead and Salmon. You can refer to this a great article by Asland Fly Shop on how to tactically make use of RIO MOW tips for swinging flies.
MOW tips were designed by some of the most influential pioneers in Skagit Spey, namely Mike McCune, Scott O'Donnell and Ed Ward. The MOW tip is named after the 3 of them.
MOW tips as part of the Skagit system were created for two main reason, consistency in depth control and consistency in casting control. In the past, people will just use sink tip of different length to fish different depth. For example if you are fishing a deep run, you may have been using a 10ft sink tip. When you next get into a shallow section, a 5ft sink tip may be more suitable. However, the 5ft sink tip will shorten the overall length of your Skagit set up and reduce the bite of your anchor. As a result you may find yourself ripping out your anchor more often than not unless you adjust your casting stroke to accommodate the change. With MOW tip, you will find that the overall length remains constant except for the fast sink portion. This consistency in length of the MOW tip results in good depth control without sacrificing casting.
An article here by Gorge Fly Shop gives a good account of not only the MOW tip but also the newer RIO iMOW tips:
Please do not confuse sink rate vs grain weight of the tips. Sink rate is ips (inch per second) whereas tungsten tips are rated at T8, T11 or T14 for example. "T" stands for tungsten coated. So a T14 means that the tungsten coated tip weighs 14 grain per foot. So if you have a 10ft T14 tip, the overall weight of the tip is 140 grain. Typical sink rate of T8 is 6-7 ips, T11 is 7-8 ips and T14 is 8-9 ips.
RO also makes tips rated in ips, for example RIO InTouch 15ft sink tips. So if you are using a 15ft sink tip rated at 3-4 ips you may be tempted to think that you are swinging your fly deeper than a person using 7ft of T14. You may be very wrong. T14 sinks at 8-9 ips so it will cut through the current much faster than a 4ips 15ft sink tip.