You want to have him feel comfortable being connected to the bit, so you apply some pressure, by kind of tightening your hand, and when he gives, you give equally. Once he feels kind of "safe" with you taking up more contact (because he knows he can get a release from that by giving in the poll) then ask for him to flex a bit at the poll and when he does, you advance your hands forward a bit and put a bit more leg on. Your aim is to have him follow your hands (actually, he is following the bit, but how he feels it coming from you hands) forward, and downward. It is important to not just throw your hands forward, and to not let him slow down and just "flump" out of the trot. You want to have more energy to fill up the widening space you are providing by advacning your hands a bit, as if you are stretching a rubber band out more. To do so, you must apply more energy, right?
More energy@! That's the ticket.