Reactions are changing a bit for the Pulp fivecore rules, particularly to make the game feel a bit more energetic and less "military".
Under the new turn sequence, half your figures will be able to react each turn (the half that didn't activate).
Reactions take place at the END of a move or AFTER any action (shooting, brawling, accomplishing a task).
This means you can slip from cover to cover without taking fire, which seems to fit the style of an Indiana Jones movie or video games like Uncharted or Tomb Raider.
In a bigger, military game that'd cause some problems ("Panzerbusch") but when running around an ancient ruin, fighting Nazi's, we've found it makes a pretty solid experience.
So how can you react then?
In two ways: By firing or by moving.
A firing reaction allows a single Shock die to be rolled (two if you have an automatic weapon).
If you fail to get any hits, the figure loses their reaction ability for the rest of this turn (a bit like Crossfire).
If you move, you get a 2" move in any direction. If you move and remain in the open and in sight of the active figure, you lose reaction ability for the rest of the turn.
The outcome is that the game feels a bit more frantic: Things are happening around you, the hero pokes around a corner and is met with a spray of gunfire or the nazi's come running down the hallway and you dive for cover.
It also helps encourage a more cluttered table with lots of smaller terrain pieces which looks great but the rules havent always supported it beforehand.