.btn-flash {
  position: relative;
  overflow: hidden;
  transition: 0.25s ease;
}

.btn-flash::after {
  content: "";
  position: absolute;
  top: 0;
  left: -120%;
  width: 120%;
  height: 100%;
  background: linear-gradient(
    120deg,
    rgba(255,255,255,0) 0%,
    rgba(255,255,255,0.35) 50%,
    rgba(255,255,255,0) 100%
  );
  transform: skewX(-20deg);
  opacity: 0;
}

.btn-flash:hover {
  transform: translateY(-1px);
  box-shadow: 0 4px 16px rgba(0,0,0,0.35);
}

.btn-flash:hover::after {
  opacity: 1;
  animation: btnFlashMove 0.7s forwards;
}

@keyframes btnFlashMove {
  from { left: -120%; }
  to   { left: 120%; }
}