Ceiling Boost
A ceiling boost is a variation of the headhit in which upwards momentum gets converted into forwards momentum instead of being canceled out. To achieve this, you need an upwards slanted ceiling to bump into.
Execution
The execution of a ceiling boost is the same as that of a headhit. Just do a normal jump but in this case under a slanted ceiling.
Bc the ceiling is slanted instead of canceling all your momentum some of it will be transformed. This can be attributed to the effect of reflection you might know from your physics class. If a vertical ray hits a 45° mirror the outgoing ray will be horizontal. The same can be applied to the y-component of Lucio's momentum and the slanted ceiling. This results in you gaining some extra speed.
Pros and Cons
As you can see, ceiling boosts are basically an evolved form of headhits that give you more speed. That's also their big advantage over headhits, and by association also oneticks. This extra speed also leads to the same disadvantage that headhits have: They're not practical for hitting jumps directly after you performed the tech. They also require a slanted ceiling.
Tips
Slanted ceilings are more common that you would be led to believe. In fact, almost every ceiling end is a slanted ceiling.