In case people weren't aware, the main ingredient in Fermcap or other Foam Control drops is dimethylpolysiloxane, which actually goes by many names. It is the same stuff that they use in anti-gas tablets (not the enzyme-active ones like Beano). It is an antisurfactant, which simply means it helps to prevent the formation of bubbles and foam.
It is actually designed to NOT make it into your beer. This is clever stuff. On its own, it is relatively heavy and does not carry a charge, meaning that it won't dissolve in water. However, with a bit of agitation (e.g., a rolling boil or active fermentation), it gets stirred up and suspends in the wort/beer. That's when it does its magic. When activity calms down, it drops out. When you siphon or rack off your beer or wort, it stays behind in the trub.
Some are often hesitant to use foam control because they are worried that it will ruin head retention in the final beer. From what I have explained above, you can see that this won't happen. In fact, it can be argued that foam control will actually IMPROVE head retention in a side-by-side comparison to beers where a blow-off tube was used to manage a large blow-off. Many of the proteins responsible for head retention get 'stuck' in the krausen during active fermentation. If much of these proteins is blown-off into a blow-off jug outside the fermenter, they are no longer able to aid in head retention in the finished beer.
I haven't done enough batches yet to confirm this benefit, but I expect to see it. Curiously to me, my only beers that had poor head retention were AG batches with lots of wheat and rye in them. However, these are well known 'head builders' because those grains contain a lot of protein (particularly the flaked varieties that I am fond of using). So why were these beers the worst for head retention? Well, these were also the ones that had monster krausens (likely because of all those proteins) and massive blow-offs. My theory is that when I do these recipes again with foam control drops, the massive blow-off will be abated, and I will see fabulous, persistent head again in these beers.
BTW, it has been asked above how much I use in the boil -- I use two drops per gallon as per the manufacturers directions. I have actually used less in some recent batches, and it seemed to have worked just fine.