Underflow(UF) and Overflow(OF) are both ways to increase skills and talents above their normal limits. There's an important distinction in that OF is allowed on all runs, while UF is most definitely not.

OF: This is done by putting skill/talent points that you already have into a skill/talent that is already capped. Video example can be seen in most of my runs, sorry for not have one on hand. It is done by quickly moving to the capped skill from a non-capped skill and pressing the select at just about the same time. This gives the option to upgrade the capped skill further, and can be repeated.

UF: Roughly similar to OF, this will instantly raise a skill/talent to rank 255 without consuming any of the player's current skill/talent points. This is done is a very similar way. You start on a skill/talent that has at least on point in it and move to one that has no points in it. Doing the same procedure as in the OF, you then select refund all instead of upgrade. Do it again but select refund one this time. This will set the skill/talent to 255. Again, UF is incredibly broken and will not be allowed in Any% runs.