Summary
Jackals use personal energy shields for protection from enemy fire. Worn on the forearm, they create a barrier that can deflect any solid projectile weapon except rockets. There are two types of Jackal personal arm shields. Ones carried by minor (black skinned in H1 and dark-blue clad in H2) Jackals are cyan. And the other type, caried by major (orange skinned in H1 and gold/yellow clad in H2) Jackals is orange. Some Jackals (snipers) don't carry shields at all.
--Note-- The shield colors from H1 are the same in H2, only better in graphics and design.
The Jackal Personal Energy Shields are a dual wield defense weapon, meaning that Jackals can only use one handed weapons with these (exception: You can get the Jackals to glitch and accept Carbines when you are the Arbiter, but they usually drop these.)
Damage
Grenade Damage
It should be noted that grenades, even though deflected by the shield, can still explode and collapse the shield if detonated nearby.The shield reflects it's current charge by changing color gradually from its original color to red before it deactivates completely.
Plasma Weapons
Energy-based weapons can penetrate the shield and can even collapse the shield altogether. The MJOLNIR armor's shield was devised by reverse-engineering the Jackal shields, and it was later improved upon it to make it much more efficient.
Conventional Weapons
For some unknown reason, some of the conventional weapons can penetrate the shields if they are fired continuously.
Halo 2 changes
In Halo 2, the shield appears to have been weakened. It is now possible, but difficult, to bring down a shield with projectile weaponry, and a sniper round to the central dot will collapse it and stun the Jackal. However, the shield now provides the Jackals with limited melee protection.