Basically HIT means it will not inflate until it is submerged 4" under water so that it will not inflate while stored in a damp storage box or if you are in heavy rain as some of the cheaper models and brands tend to do.
NOTE: There is a safety advisory on these models from Mustang if you have one that was manufactured during April or May of 2012 in Canada. You can find this safety advisory on the Mustang Website:
www.mustangsurvival.com by clicking on the Learn More button of your particular model. You can also check the inside label of your vest to see when and where it was manufactured. Also, read the instructions that come with the vest and perform the self inflation test to ensure there are no leaks in the vest!!