Late Skim
A lateskim, as already mentioned, is almost the same as a latejump. Lateskims are used for bridging longer gaps as well, using the same principle as latejumps. As their name suggest however, they are derived from skims instead of jumps.
Execution
With this, their inputs are almost identical to those of latejumps. The 2nd jump input is delayed to the maximum again. The first jump input, contrary to latejumps, is reduced to a single tick, just like with skims. Therefore the name, lateskim.
Same as with latejumps, the delayed wall jump gives you the extra length this tech provides. And just like with skims, the one tick long wallride ensures that you lose close to no speed.
Pros and Cons
This preservation of momentum is also the big advantage lateskims have over latejumps and why they're preferred in almost every situation. Together with skimming they are the fastest method of moving around the map. Just like with every tech, lateskims are not perfect for all situations. Their harder timing makes them unsuitable for perfect consistency and their short wallride time makes turning corners harder.
Tips
Lateskims, are the most important part of Lucio's kit, next to skims and bhops, a tech we'll turn to shortly. Using scroll wheel to input a single tick can help with getting the one tick wallride down, although scrolling only one bump might be hard to do.