It's not an incredibly powerful effect, but it is still useful well into the later stages of the game. You get more SOUL for hitting enemies, which is useful for casting spells and healing. The description on this charm is pretty self-explanatory.
![hollow knight map guide hollow knight map guide](https://oyster.ignimgs.com/mediawiki/apis.ign.com/hollow-knight-wiki/e/e9/Mask_Shard_3_Map.png)
This charm can be more effectively used with the Quick Slash and Fragile Strength charms to output a ton more damage, or the Quick Focus charm to get a quick heal during the invulnerability. Mostly useful in boss fights or against large groups of enemies that are hard to dodge, the charm gives some extra time to escape a situation or start swinging wildly. This charm increases the length of the invulnerability period that occurs after taking damage.
![hollow knight map guide hollow knight map guide](https://cdn.holdtoreset.com/wp-content/uploads/2017/03/hollow_knight-2017-03-15-23-47-13-460.jpg)
When overcharmed, play carefully - as you will take double damage until you are no longer overcharmed. If you attempt to equip a charm that uses more notches than you have, the game will refuse a few times before finally relenting and equipping the charm. A risky concept, the ability to be "overcharmed" allows you to equip more charms than you should be able to.
![hollow knight map guide hollow knight map guide](https://cdn.vox-cdn.com/thumbor/535_ntMMOcPpdDDK4C-_c9DbVuk=/0x0:2742x2907/1200x800/filters:focal(1242x1533:1680x1971)/cdn.vox-cdn.com/uploads/chorus_image/image/60421511/hollow_knight_guide.0.jpg)
The notches that determine how many charms you can equip are not a hard cap.