It is a special potion that you can obtain from monster park using 5 coins obtained from doing monster park. This potion has the unique effect of breaking the speed cap of 2 and allowing you to get to the hard speed cap of 0. In short if shadowers are at the soft speed cap of 2 with booster, then if you use a green potion you will be at a speed of 1 and then using dsi puts you at the hard cap of speed 0.
Daggers are attack speed 4. If you use booster you hit the soft attack speed of 2. DSI does nothing to break softcap but the MPE green potion can break attackspeed 2. If you use MPE green pot you get attack speed 1. Then throw in DSI and you get attack speed 0. DSI is useful to shadowers but only if you use green potion-similarly to Dual Blade.
Booster on its own will net you soft cap att speed 2 (since dagger is attack speed 4 and booster is an increase of 2 stages). You need MPE green potion to break soft cap attack speed so your other attack speed buffs can continue affecting the attack speed. Shadowers benefit greatly from it as they gain attack speed increase on assasinate, bstep, and dash (for dodging) and just makes the class much less clunkier with increased dps. As a node it is actually okay to use it because sometimes in bossing it is hard to switch out equips with decent skills, while using it as a node makes it much easier to consistently use to the ability of hotkeying the skill (which is important because endgame bosses usually last longer than a decent skill duration). In terms of CO vs SI, CO wins however, because the attack speed bonuses get outweighed but the damage increase bonuses of CO. Also the difference betweent soft cap attack speed and hard cap is not THAT big, its just noticiable.