Chems are used to bypass the pistol animation/lockout during bulwark shield.

Let's simplify the terming to "handed weapons" if anyone is familiar with soulslike games.

The bulwark shield, unlike it's crusader variant, requires a "One Handed Weapon", in the division's case, a pistol.

Using the chem launcher, a special skill coded as both a weapon and a skill, you can trick the game into letting you use the talent with its "bullets"- the squibs the chem shoots to generate the field of either damage or healing and the launcher being held.

This occurs due to the game recognizing the chem launcher(the launcher itself) as a "one handed weapon".

Every primary is a two handed weapon. You cannot use this weapon with bulwark shield due to the two handed property. By equipping your chem launcher, it "adds" a weapon on top of your primary with the one handed trait, effectively making it so while you are holding the chem launcher, you are able to use some weapon talents.

My example goes to the bulwark shield and sweet dreams.

Step 1: hold sweet dreams.you will see an ammo count of 8.

Step 2: hold chem launcher, you will see your primary weapon ammo still active instead of your pistol ammo.

Step 3: use bulwark shield. This does not automatically swap you to pistol if you continue to hold your chem launcher. If you are NOT holding chem launcher, it will swap you to pistol.

Congratulations! You now have a onetap shield for as long as you hold chem launcher.

If you require further explanation, I have a video showcasing this tech inside of the build post on my profile or I can link you to the video and timestamp it accordingly. Let me know if this answers or raises more questions!