The snowflake is for descend vias. Its amazing for those actually. The snowflake "looks through" all the at or above, and at or belows and guides you through them. At XJET we are taught that if you get a descend via, you set the next lowest "hard" altitude (not the at or below, or at or above, but the first "at" only altitude). When you hit that hard altitude keep setting the next "hard" altitude. Then you look for the TOD on the MFD, and when you approach the TOD, the snow flake will appear and you descend to match that. There is no vertical speed descent rate to follow to ride the snowflake down, you have to guess and see what allows you to keep up with the snowflake. A very common mistake is thinking the green data block on the PFD correlates to the snowflake, it does not. The descent angle and the vertical speed that is on the FPD (the green "data block") is ONLY for the the next hard altitudes on the FMS, and does NOT comply with the "at or above" or "at or belows" and will get you into trouble. If you follow the snowflake you will cross usually towards the top of the "windows" on the descent vias. You also have to watch the snowflake like a hawk to keep it from getting away from you. If you have a strong tail wind on a descent via while using the snowflake you have to be mindful of your groundspeed as use of the spoilers will probably have to be used as your descent rate can get high, especially if you have a slow down associated with your crossing restrictions on the descend via. If ATC vectors you off of the descend via, ALL the data and snowflake info goes out the window. ATC often is not aware of this, so you have to tell ATC that you need a new altitude to descend to, as you could get into trouble because the FMS does not know how to guide you down. You are now kind of descending into nowhere scenario.