Get Fired Up
This is good instruction, but to be effective, it must be applied correctly. Two of the reasons I'm wary of the "fire" concept are that it (1) is sometimes misunderstood and (2) is often performed incorrectly.
The misunderstanding happens because of the human architecture. Since we are designed for walking and running, biomechanically the knee flexes over the foot, a position that aligns the hip, knee and ankle joints to provide maximum stability. Thus the natural (but incorrect) tendency during the downswing is to move the back knee out toward the target line as if you were going to step in that direction.
Phil Mickelson is an example of a player who fights this problem, and it is just another indication of how counterintuitive this game is -- what is natural is wrong. As Jack Nicklaus says, "Golf is best played on the inside rims of your feet" -- a departure from the normal that must be learned.
When you fire your trail side correctly, your back heel is off the ground, and as part of the release, your back shoulder moves from a high position behind your neck to a low position at impact while your trail elbow moves in front of your hip.
For some excellent slow-motion swings that demonstrate the correct way to fire your trail side, go to: www.youtube.com/watch?v=d_axF1OpSzA.
If you watch these swings, make sure to stay for the last swing, which is actor Morgan Freeman. While the pros show the correct move, he demonstrates the wrong move as his right side remains passive. Stop the tape at impact and you'll see he's completely "unfired."