Look carefully at the graphs. Flip flops seem to allow for less peak dorsiflexion of the foot (plantar flexion needed to hold the flip flop on?) and more inversion and eversion of the foot. Makes sense since there is no heel counter to stopthe calcaneus from inverting or everting.
“The results from this study indicate that barefoot, flip-flops and sandals produced different peak GRF variables and ankle moment compared to shoes while all footwear yield different COP and ankle and knee kinematics compared to barefoot.”