A dive finger or jump spool is the most compact way of carrying a guideline. A finger spool makes deploying a delayed surface marker buoy (DSMB) very easy as there is no way for line to get caught in any sort of mechanism. Jump spools are also useful for make jumps and gaps in caves. They are also a handy way of mapping a dive site as they are quick to deploy and re-stow.
If you're looking for a dive reel, please see Dive Reels.
Finger Spools Are Not Guideline Reels
The skills required to deploy a finger spool are different than those required to use a typical guideline reel. At first glance the simple spool looks easier, but compared to guideline reels the finger spool requires specific techniques for successful use. If this is your first finger spool then seek advice, get a demonstration, and most importantly you should practice under controlled conditions.
A common usage mistake is rewinding a finger spool in a manner that applies a lot of tension to the line, such as when hanging under a lift bag, causing the line to be very tightly wound on to the spool. Upon re-use the line doesn't smoothly unwind and sticks, causing the entire spool to be jerked out of the fingers or disappear with the lift bag.
Another consideration is the method used to rewind the line on to the spool. Hold the line loosely and use the spool to take up the line; if you hold the spool in a fixed position some wrapping motions with your hand will put a 'twist' in the line with each wrap around the spool. Depending on how you later unspool, you might wind up with a tangled mess of line (aka 'birds nest') if you hold the line in your hand while it unspools.
Although your finger spool is not a guideline reel the basic care is very similar. You should rinse your finger spool with fresh water following each dive. We recommend pulling some or all of the line off the spool before each dive, then rewinding it neatly but not too tightly, so as to help prevent problems during line deployment. If the line on your finger spool is new or very dry, we recommend pulling the line off into a bucket of water. This will moisten the line, precluding it from swelling on the spool and causing further issues or damaging the spool.
Finally, too much line on the spool can cause tangles during initial deployment, if you are having a consistent problem deploying your finger spool try removing a few metres of line.