The so-called “whoosh effect” actually has nothing at all to do with having soft squishy fat, on your belly, or anywhere else. What this refers to is when you are first starting to lose weight, and burning fat cells, often your body will retain water instead, so that your weight stays the same when you step on the scales each morning.
Eventually, your body will flush out all of that excess water (especially if you have only protein for that day) and then you will seem to drop several pounds overnight.
Actually, you have been burning fat all along, and have only lost water weight, but it still can make a dramatic change in how you feel and how your clothes fit.
As far as having soft flab at your stomach area, I think that
@DenisP is absolutely correct, just exercising more should help to build muscle and firm up your abs.