Acceleration shots are the most powerful of the shots available to you, and are primarily used to get on the offensive in a rally or to go for outright winners - they take more of your stamina bar away than the regular strikes, and this is affected by the style of your character (Power Baseliners can use more accels than Volleyers with less tiredness penalty). They are mapped to "B1+B2" in your controls, which can be seen and remapped in the control options menu.
The different control settings (arcade/sim/elite) affect the difficulty of shot execution and frequency of errors. Elite is the most difficult, and is very unforgiving if you are just slightly outside of the "sweet spot" for striking the ball (especially on accel shots) and shots need longer to charge for the same level of precision as the easier difficulties.
The game documentation explains these things in greater detail and I think it's worth reading to gain a better understanding of some of the gameplay mechanics;http://www.managames.com/tennis/doc/Ten ... _Game.html