IMSI is a modification of the ICSI technique where sperm samples are examined under a microscope that is almost 6000 times more powerful to better assess their "morphology". At a higher power the embryologist can identify tiny defects in the sperm head that would not otherwise be visible with standard ICSI.
Although it is yet to be confirmed to be of benefit for all requiring sperm injection, it is recommended in those with a history of recurrent implantation failures, recurrent ICSI failures or severe teratospermia (abnormal sperm shape).
In essence it is a selection tool, to aid embryologists choose the optimal sperm for ICSI. If the sperm count or motility is already so low that it limits the number of sperm available for ICSI, IMSI may not be a suitable technique for you.