In past releases of AutoCAD you could develop a nice Dynamic Block but had to save it to see if it worked and then re-enter the Block Editor (BEDIT command) to tweak and tune it, and repeat.
Now with AutoCAD 2010 and AutoCAD LT 2010 in the Block Editor there is the ability to test your block speeding up the creation of powerful Dynamic Blocks.
You can access this new Test Block option from the Block Edit ribbon or using the AutoCAD command BTESTBLOCK when in the Block Editor.
Cheers,
Shaan