Clasp assemblies that accommodate functional prosthesis movement are designed to address the concern of:
Clasps Designed to Accommodate Functional Movement
Clasp  assemblies  that  accommodate  functional  prosthesis movement are designed to address the concern of a Class I lever. 
The concern is that the distal extension acts as a long “effort  arm”  across  the  distal  rest  “fulcrum”  to  cause  the clasp tip “resistance arm” to engage the tooth undercut. This results in harmful tipping or torquing of the tooth, which is greater with stiff clasps and increased denture base movement. 
Two strategies may be adopted to change the fulcrum location  and  subsequently  the  “resistance  arm”  engaging effect  (mesial  rest  concept  clasp  assemblies),  or  to  minimize the effect of the lever through the use of a flexible arm (wrought-wire retentive arm).
Key Concept:
Clasp  assemblies  that  accommodate  functional  prosthesis movement are designed to address the concern of a Class I lever. 
Ref: McCracken's REMOVABLE PARTIAL PROSTHODONTICS, THIRTEENTH EDITION page no 72
Class I lever
Class II lever
Class III lever
Class IV lever