Pre-drilling is, more or less, for normal screws. A normal screw cannot cut into the wood, it merely pushes it out of the way. But you're buying the nice, pricey screws. Drilling? Where we're going we don't need drills! Just grab your impact driver and go to town! That's why you paid $20 for 12 screws, right?
I rather like Harper's point about lag screws. Lags do far better in this situation, but it's worth talking about something else important (screw or lag): technique. Most likely you put the screw in your driver and started driving. There's two things to keep in mind on screws of this type:
- It's still generating debris (sawdust)
- It's still generating heat
Your impact driver is an amazing tool. I remember buying a power screwdriver about 20 years ago and it wasn't anywhere near as versatile. The "impact" part vibrates the screw as you're driving it, which helps lower resistance. And for short self-driving screws, they pairs perfectly with an impact driver. Those 3" screws go in with ease! But the longer the screw, the more you need to "work" the screw, especially if you hit something tougher in the wood (like a knot).
At this point, the old Tim "The tool man" Taylor instinct comes out (that's why the character is so funny, because we all can relate to that) and you start to think "We need more power! Lean harder! Squeeze that trigger like your life depends on it!" Don't do that. You do need consistent pressure, but pushing too hard gets you where you are now: the screw is stuck and bent. You might also have rounded the head some, if you weren't applying enough pressure. Getting that out will be fun.
Listen to your screw
When a screw stops turning, it's a good idea to stop. The longer the screw, the more likely this is to happen. What you might want to consider is backing up and trying to slowly work it back down. The nice things about impact drivers is they can provide amazing torque. That usually translates into speed, but for long screws we don't want speed, we want it in. Backing up and going forward some can help work the tip (which is doing the job of a drill) further in.
If it binds up on a slow retry, I would suggest backing it all the way out. This will help clear some debris from the hole. You can then let your screw cool some. Then try again slowly (remember, you have a shaft now so the tip won't get as hot). Pre-drilling just saves you whatever heat and debris the length of your drill bit can take out first (which probably isn't a lot on a narrow-shaft bit). Some of your screws may go in without complaint, but some won't (especially on a 4x4, which is a lot more likely to have knots). Working the screws in a slow but deliberate manner can get you where you want to go: screwing without drilling. Trying to force it seldom goes well.