Mostafa --
I might suggest reading through the Overview of Thin Image in your Thin Image User Guide (manual MK-92RD8011).
HTI does not stop host IO. Changes are stored in a pool (several options here including a dedicated HTI pool, the same HDP pool that the primary device is in, or a separate HDP pool). HTI is a Copy After Write snapshot technology; we acknowledge new writes, holding that data in cache while doing any necessary data movement on the back end. Every primary volume is capable of having up to 1,024 snapshots, and a G1000 is able to support 1M snapshots total. There is no maximum retention, but a snapshot may not be the best solution for a long term need.
Application awareness is accomplished via integration with a host agent (e.g., Hitachi Data Instance Director will take application aware snapshots using VSS to quiesce SQL). The application stays available, but I can't say there is "no impact" while changing any application to/from hot backup mode.
As for "risk" of growing change, if the pool is not sized to handle the change your snapshots will suspend in an error condition. Production will be unaffected, but the snapshots will not be valid.