Ever hear a fly angler mention "smacking" or "slapping" the spool? It sounds kind of strange, but it's actually a really effective technique to gather up loose fly line extremely quickly—if done correctly!
There's two ways to fight a fish with a fly rod and reel: you can either strip a fish in without ever touching the reel, or use the reel to crank the fish in. Simply stripping a fish in is an easy and quick method. You don't have to reel up any loose fly line first, nor do you have to strip line off the reel again when you're ready for a new presentation. You're not using the reel's drag at all, but depending on the fish and your equipment, that's not always necessary.
On the other hand, fighting certain fish from the reel is much safer since you can utilize the reel's drag system. When a fish runs, line pulls from the drag smoothly (hopefully!) which protects the line and hook from sudden jolts. It's also cleaner since loose line isn't all over the place. However, there's two downsides to this method: first, when it's time to cast again, you'll have to re-strip all that line off the reel— not that big of a deal. Second, not all fish will "get on the reel" for you right away. What does this mean?
To get a fish "on the reel" simply means to get rid of all loose line and fight the fish directly from the reel itself.
Let's say you're stripping a fly on your favorite stillwater and a big brown trout eats it about 3/4 of the way in. The hook is set, and the fish immediately takes off directly away from you. As the fish runs, you hold the rod up and let all the slack line slip through your fingers as best you can until it's all gone. With all the slack gone, the line is now coming directly off the reel itself. This is a very good thing when you're hooked to a big fish and/or are using light tippet. That fish just got "on the reel" for you, which quickly put you in a more advantageous position since there's no slack to deal with and the reel's drag can now be utilized.
The problem scenario occurs when you hook a fish you'd prefer to fight from the reel, but it initially runs either towards you or just lollygags around not really taking any line. You can start fighting that fish by stripping the line to maintain pressure and just let the loose line pile up even further, but that slack can eventually tangle or pick up debris. Also, if the fish makes a lighting-fast run, the line slipping through your fingers means uneven pressure on the line and can result in a pulled hook or popped leader. For these reasons, if a big fish doesn't quickly get on the reel for me by running, I typically want to get it on the reel ASAP both to protect the line and so I can follow it more easily if needed. I do this by quickly reeling in the slack or by smacking/slapping the spool.
Slapping the spool can work great, but it can also really foul things up if done sloppy. I prefer to use it only in certain situations, especially when I want to get a fish on the reel without delay!
Slapping the spool is pretty straightforward. With the fish hooked up, the stripping finger(s) of my rod hand pinches the line firmly against the grip to maintain pressure on the fish. With my free (reeling) hand open and 90-degrees to the reel, I quickly slap or flick the rim of the spool with my fingers, typically in front of where the line is dangling off the spool. Doing this makes the spool spin like crazy and takes in lots of line extremely fast. As long as the line will feed on properly, the spool can be slapped repeatedly to gather up as much line as desired.
There's a few points I'll mention regarding this technique:
Not for Every Reel: Keep in mind that not all reels work well for this. Some reels have a "tighter" spool and don't spin much when slapped, while others are "looser" and will spin several revolutions per slap, thus making them much more effective.
Line Management: Before slapping the spool, I take a quick look at the loose line to make sure it's not tangled up or fouled with debris. If the line is messed up when I go to slap the spool, that tangle or hunk of debris will likely go right into my spool. If there's an issue, simply stopping and giving the loose line a few quick shakes can often work out any kinks or free it of any junk. Keep in mind this can happen at any time while retrieving the loose line. Managing this loose line properly by keeping it from piling up and away from potential hazards helps a lot.
Pay Attention: It's easy to get fixated on the loose line, but remember to stay vigilant on the fight, too! Keep ample pressure on the fish, and be ready for a sudden surge. If that happens, you'll want to quickly grab the line with your free hand and unhook it from your rod-hand finger so you can more easily let line slip out. Keep in mind, too, that as you slap the line back onto the spool, you may have to stop at any time to strip the fish in a few feet or make other necessary adjustments. Fish don't often sit still!
Less Line the Better: Personally, I often prefer using the slapping technique when I don't have a ton of loose line out. Why? There's less to go wrong. If there's only 15 feet of slack line out, it's less likely to be or get tangled or be under much tension from water/current than if there's 50 feet of it strewn out. When I do have a large amount of loose line out, I'll usually take the slower but safer method—hooking it around my rod-hand pinky and guiding it back onto the spool as I crank rapidly. This puts the loose line back onto the spool cleanly and evenly, and I can feel for tangles or debris with my finger as I reel. Sometimes I even use a combination of both methods!