  We consider the reflection of relativistically strong radiation from plasma
and identify the physical origin of the electrons' tendency to form a thin
sheet, which maintains its localisation throughout its motion. Thereby we
justify the principle of the relativistic electronic spring (RES) proposed in
[A. Gonoskov et al. PRE 84, 046403 (2011)]. Using the RES principle we derive a
closed set of differential equations that describe the reflection of radiation
with arbitrary variation of polarization and intensity from plasma with
arbitrary density profile for arbitrary angle of incidence. PIC simulations
show that the theory captures the essence of the plasma dynamics. In
particular, it can be applied for the studies of plasma heating and surface
high-harmonic generation with intense lasers.
