Reclaiming unused space through alter table shrink..,after mass insertion/deletion from table

Dear All,

I have found something about table shrink to reclaim unused space , after mass insertion/deletion from table....

So My queries are,

Oracle DB - 11.1.0.6.0 Standard Edition

On Internet, I have found something like below queries to shrink table

  " ALTER TABLE TABLE_NAME SHRINK SPACE COMPACT" 

and after that apply  following command

 " ALTER TABLE TABLE_NAME SHRINK SPACE"

Why we should apply compact query first and then shrink space ? what is the effect of it?

and also to use shrink space, you need to enable row movement.....

What is row movement? and What are the effects on database (performancewise etc...) if row movement is enabled?