The trident is held (in first person) in a way that looks like it is being held from lower down. It also makes the trident look thinner than before. The main side effect is that when attacking with it, the animation looks similar to when you have mining fatigue, and once the trident reaches the end of the attack, it just teleports back to the normal "being held" position. This effect can also be seen when switching to the trident, as it teleports into your hand, goes down, and then back up.

To replicate, hold a trident. You can also switch to it or swing it to see the side effects.

Java trident included in screenshots for extra comparison.