Hybrid load pull refers to a combination of active and passive tuning within the same system.
In essence in such systems traditional passive mechanical tuners are used to prematch the device allowing a much smaller active injection signal, using much smaller amplifiers, and thus reducing cost, to overcome the system losses and achieve GL>= 1.
Since the power levels of harmonic frequencies are often well below the power of the fundamental signal, lower power amplifiers can be used with active tuning to accomplish active harmonic load pull. Both cases require only low power levels for active tuning. One thing to remember is that such systems typically have the same problems of uncontrolled out of band impedance presenting potential stability issues, although this issue is only relevant to certain devices and technologies.