it will work as long as you are the one initiating the connection. if some peer suspects you have a wanted piece available i.e. from another peer in the swarm i

it will work as long as you are the one initiating the connection. if some peer suspects you have a wanted piece available i.e. from another peer in the swarm it can not communicate the intent to get that piece from you to your client directly. i think BitTorrent can relay messages through intermediate peers to make your client establish the connection to that other peer (reversing the initiator). Otherwise peers will exchange other peers that are visible to them so that your client might eventually learn how the other peer that wanted that piece is reachable and connects to it. So it actually will work without port forwarding but reaching your client will be harder and thus less peers inside the swarm will be available to you or them, likely making it slower.