When I see something in the 1st 3 Q's then I will consider changing my tune. When a team is winning on late game heroics, it doesn't bode well for them. Also,once there is enough film, it can go south fast. See Josh Freeman. See Bradford. Right now, he is terrible. Block out the 4th Q vs prevent Ds (tebowners need not try) watch the play, and ask yourself if you want this guy leading your team.
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I think this is a pretty important question and I wanted to take some time to answer this. I agree - I'm not happy with the level of play thru Q1-3 and I want to see 60 solid minutes of football before I'm ready to go all in...I think Elway wants to see the same thing too. I will also agree that the way the Broncos have been winning is not sustainable year after year.
There are a number of factors that lead the Broncos to have success almost exclusively a the end of a game. I don't think one can empirically say exactly what it is, just identify possible causes and that's what I'm aiming to do here.
[*]I once had a coach say that the way you beat a better opponent is to keep it close and win in the end. keep it conservative, keep the score low, don't make any mistakes, and then go all out at the very end. I think that is exactly what Fox is doing. I also happen to think that is how Reeves played Elway back in the day; it was good enough to take the Broncos to 3 SB's, so IMO it's a proven strategy.
[*]Fox has claimed that the Broncos get out to a slow start because teams by now have film on the Broncos, but the Broncos don't have film on how other teams will defend them. Essentially, when you are running a unique, one-off offense, you are at a disadvantage, everyone knows what you want to do but you don't know what they want to do. It may take a half or more to figure this all out.
[*]I think that the way you beat Tebow and option-based offenses is actually simple - play disciplined, assignment football. If players stay home, read-option will not work, that much is simple. Further, if DB's stay on their man instead of peeking into the backfield, guys won't be wide open. If coaches stick to what has been working, it will continue to work. However, late in games, I say discipline breaks down. Players are tired - more likely to play off of instinct instead of maintaining responsibilities. Coaches start doing silly things like abandoning 8 in the box defenses in favor of deep cover-2.
[*]Broncos run a power offense, which will eventually tire a defense down (ref: Hairy Snowman's post). This is compounded by playing home games at 5280' elevation. Further, when Broncos go no-huddle, defenses don't get a chance to rotate out.
[*]A change in Broncos personnel groupings and strategy when the game is on the line. It seems that thru Q1-3, Broncos run lots of 2TE, 2 RB sets - power football. Tebow only throws on third and long - not really a good passing situation, no? The Fox offense is run, run, throw if you have to...it's really pretty predictable, and may not be suited exactly to Tebow's (or any QB's, for that matter) strengths. It may be a good way to win football games, but not pad stats. There are folks out there right now crying that Elway/Fox continue to run this offense deliberately and consciously to sabotage Tebows development, and harp on Tebow's early game throwing situations incessantly. (go to orangemane.com if you want to see proof...couple of gator homers are ruining thread after thread complaining about this). On the other hand, in Q4, Denver goes to a spread - 3WR, 1 RB, one other WR or TE, and ask Tebow to win out of the shotgun. This is the personnel grouping that Tebow thrived with in Florida and really does play to his strengths. Take a fish out of water, he flops. Put him back in, and watch him swim!
This article has better detail of what I'm trying to say.
[*]Some folks thrive under pressure, some folks fold. Tebow may be the most clutch player in pressure we have seen come along since Joe Montana. I'm talking about specifically Tebow's ability to elevate his own play in the face of adversity.
[*]As much tactics as there is in football, it is an emotional sport too. People feed off of each other. People inspire each other. I truly believe that Tebow makes his teammates play harder, Elway basically said as much earlier today. When guys are in the huddle and see Tebow, they believe. If you believe, you can do. Defenses feed off of this as well. They know that if they can give Tebow a chance, he will come thru.
[*]Conversely, opponents feel the opposite. By now they know that the Tebow comeback is almost inevitable, and play scared. When you play scared, bad things happen.
[*]Dumb luck.
Well, that's all I've got for now. I'm not saying the answer to your question is all of the above, or none of the above, or any combination thereof. These are just some possible reasons for the late-game dramatics in attempt to rationalize what appears to be irrational.