A fixture studied with an intentional resonance between shaker and item is called “resonance fixture” and it is usually designed in order to achieve a very high vibration level.
When the test to be performed presents a very high level in a restricted frequency range the only factual method to achieve the required level may be represented by the use of a resonant firxture.
The item is mounted in a resonant part of the fixture tuned to resonate at the frequency of interest. The amplification factor of the resonance fixture allows to achieve easily vibration level of hundreds of g.
The use of a resonant fixture can save a lot of energy and prevents shaker damages. Many times it is the only way to achieve the required test level.