Reflex/Deflex/Straight risers basically apply to where the throat of the grip is compared to the pivot point of the limbs.

As an example, thePSE Money Makerbelow has adeflex riser, with the throat of the grip being in front of where the limbs connect to the limb pockets:



The X-Force LD has a straight riser:



And the X-Force has a VERY reflexed riser:



If given the same limb length and angle, the deflex will traditionally decrease your power stroke (making it more forgiving but slower), but also with the deflex riser will typically settle down faster after the inititial draw/anchor sequence, and also will be more forgiving of any twitches or bad shot execution.

Hope this helps!