The controller should has a "aggressive" trim adjustment - Ie how quickly the brakes are activated. On my old controller I used have to adjust this trim for slow speed driving versus interstate driving (it became a pain); I bought a more modern Prodigy controller which has a much better inertia-style trim adjustment (although you can override it);
The controller senses how fast you are slowing and applies progressively more brakes to the trailer WITHOUT snatching.
Not sure how old your controller is but you may want to get a new one as they have come a long way..... BTW - as others have said, you may want to make sure your brakes are adjusted correctly.
