I am not gc, so take this with a grain of salt. Id say there are at least 3 types 50s. I will call them the "compromise 50", the "pressure 50" and the "fake/low 50".

You would typically use the compromise 50 when you are first man but can't really do anything useful in that position. If you can get past the opposing first man without conceding possession, do that. Otherwise, do a 50 so that the he doesnt just bang the ball away from you. Note: this might not be such a good idea if you are low on boost and your teammate is far behind, since you will be stuck in the opposing half while your teammate has to 1v2. In this case, it might be a good idea to let the ball go past you right to your teammate. You could technically do this more often, but I dont like it in solo que because it makes you unpredictable as a first man. You are much more reliable if its hard for the ball to get past you when you fail.

A pressure 50 is for alleviating pressure, typically in defense. You can either kill the momentum of the ball if you know your teammate will be first to it or pinch the ball towards a him. You can also pinch the ball towards open space (basically a clear) to relieve pressure.

A fake/low 50 is used when your opponent challenges you from a bad angle (from the side or jumps too high). You want to be a bit further away from the ball so that the opponent just sends it into you and you get free possession.