Sound is produced according to the same principles as on the oboe:
The double reed is placed between the lips and blown which causes both reeds to vibrate against each other. They open and close very rapidly, sending bursts of energy into the air column inside the instrument and causing it to vibrate in sympathy. A good English hornist is one who can do this in a controlled and sensitive way.
Because it has wider reeds than the oboe, attack on the English horn is easier. Conversely, its larger size means response is slower, making it a little more ponderous.
As on all woodwind instruments it is primarily the keys which are used to produce the various pitches. Like all oboes, the English horn overblows to the octave. The speaker keys make overblowing largely redundant. The same fingering is used on the English horn as on the oboe, but the pitch is a fifth lower.