A Dash of Everything
Dashing allows you to make your character cover a fixed distance quickly. You perform a Dash by quickly tapping either Forward twice to dash forward or Back twice to dash backwards. Every character has small differences in the speed and distance of their dashes. But oddly, almost every character consistently walks backwards faster than dashing backwards, so the usefulness of back dashes is still in question.
Dashes are extremely flexible in Mortal Kombat 9. There is almost no commitment when you perform a Dash. You can cancel a Dash at any point into a Normal Move, into a Special Attack, or -- most importantly -- into blocking. In fact, you'll probably find the majority of your forward movement will come from dashing forward and blocking, moving very short distances at a time to give yourself a much safer approach. The only thing you can't cancel a Dash with is a Crouch or another Dash.
You can also cancel Dashes into a jump, but in a limited fashion. At any time during a Forward Dash, you can cancel that into a Forward Jump and only a Forward Jump. Straight Up Jumps and Back Jumps are not allowed. And, when performing a Back Dash, you can only cancel that into a Back Jump. Straight Up Jumps and Forward Jumps are not allowed. This was probably a safety measure to prevent easy combos of Forward Dashing into Straight Up Jumping punches for ground bounces.
Wave Dashing
Thanks to the ability to cancel a Dash into blocking, however, you can actually perform a "Wave Dash" or sorts. Wave Dashing is a technique used in many fighting games to chain together a sequence of dashes. Although every game's technique is different, it's almost always universally referred to as Wave Dashing.
To perform a Wave Dash, you basically blend each tap of the controller direction to act as the first tap AND the second tap of a Dash code. To do this, start by performing a regular Dash by tapping the controller Forward twice. Now, mid Dash, tap the Block button very quickly and make sure you are not holding it down anymore. You'll notice now that if you tap Forward once on the controller, you'll immediately dash again. What happens is that the second tap of the previous Dash acts as the first tap of the next Dash. So now, you can repeat this pattern by repeatedly alternating between tapping Block and tapping Forward on the controller.
To get the proper speed boost, however, you can't do it too fast. Too fast, and your character won't really move forward that much faster than normal. However, if you get the rhythm down just perfectly, alternating between Block and tapping Forward, you can scoot across the floor very quickly. Also, some characters just don't Wave Dash well at all. Reptile, for example, dashes very slow and short at the start of his Dash, so performing a Wave Dash almost gets you nowhere. You're better off walking or doing full Dashes.