Hybrid launchers have a real stupid name, but that's what people have named them, so I guess that's what I'll be stuck describing them as.
Hybrids work by increasing the density of the fuel/air mixture in the chamber, creating more chemical potential energy available to propel the projectile.
The most common way to do this is to create a pressurized fuel/air mixture in the chamber, held back by a burst disk. Other ways include oxygen enrichment, which allows more fuel to be burnt in the same volume.
Hybrid launchers are slightly more dangerous than other types because of the stepless increase of combustion pressure made possible. For instance, 20 atmospheres of pre-ignition fuel/air compression will generate close to 3,000 PSIG in a closed chamber disregarding heat loss. In practice, this pressure will be lower in a cannon, but still significant.