Taking advantage of empty slots
It’s very common to see CFOP solvers use empty slots during F2L to make their pairs in less conventional ways, but it’s less common to see it done effectively
I recall many times I've critiqued someones solves, and the BIGGEST thing they're "doing wrong" with their F2L, is how they use empty slots
This typically happens early in a solve, as towards the end of a solve your empty slot choices approach 0
For 1st or 2nd pair however? You've got options on OPTIONS
So. many. options... yet it's rare that a cuber will really understand what the best option is, and even rarer that they choose it
Simply put, if you're not taking advantage of empty slots, they're taking advantage of you
5 things you should keep in mind when taking advantage of an empty slot
1. Save rotations (think doing U' R U R' U' F U' F' instead of y' R U R' U2 R U' R')
2. Fill up a better slot for look ahead purposes (the above example is good for this too, since the first solution fills FL whereas the second would fill FR)
3. Cut down on moves (think doing R U R' L U' L' instead of U2 L U L' U2 L U' L')
4. Give yourself a preferable move-group for execution purposes (think doing U’ R’ U R U R' U' R instead of L' U' L U R' U' R)
5. Set yourself up for a continuous flow of turns (think doing y U' R U' R' U R' U' R instead of R U' R' y R' U' R)
Notice that with the 5 above examples, although I’m comparing empty slot solutions to normal solutions, the empty slot solution ISN’T always better, and that’s the point!
Also note that in a real solve and in most random F2L situations where you have several empty slots giving you options, sometimes the "correct" answer won't be obvious
Maybe one solution allows for a continuous flow of turns and is low move count, but for look ahead purposes it fills up a bad slot and not your favorite move-group (RULgen for example)
Maybe the other solution has a move group that you like better and fills up BR instead of FR for example, but the movecount is higher and it has a rotation mid algorithm which breaks up the continuous flow of turns
Now, one way you could assess which of the two is better is by compiling a list of pros cons and neutrals between the two
And doing this isn't a bad exercise per se. It'll definitely get you thinking critically about what you're doing and how you're doing it, but...
...there's a better way
A way that isn't as time consuming
Or as lame
Or as ineffective, quite frankly
Folks, the ONLY way you're going to know which of the many hypothetical solutions is fastest for YOU, is by timing them
Do an avg25 with each different solution
See which one had the fastest avg5
And of course, make sure you're doing your timer tests properly and keep the starting AUF and grip CONSISTENT
Lastly, don't start and stop the timer while holding the cube unless you know you're not the type to do the last 3 turns after stopping the timer
You aren't trying to break any PB's or anything, so saving the extra milliseconds from picking up and dropping the cube isn't doing you any favors (if anything, it's hurting you)
Timing your execution truly is king
When in doubt
Time it out
Still, if there's an F2L solution involving an empty slot that ticks all 5 of the above boxes? It's probably the one, and you don't need a timer to tell you that (for example things like R U’ R2’ U’ R or R U2’ R2’ U’ R or even D R U’ R2’ U R D’)
Hell, 4/5 is good enough to skip using the timer
It's only when it's that murky 3/5 or 2/5 zone that the timer can really give you proper clarity
Over time you'll probably even figure out which of those 5 things matters the most
Maybe you're surprisingly good at RULgen turning, and having and LUgen solution instead isn't exactly a big deal
Maybe you're someone who can do a y2 rotation in .2 of a second or less (yes, these people exist), and saving rotations doesn't really help you all that much
Only you can figure these things out for yourself