Important note:
A correct Core Training should support your body stiffness during each push up, although this is correct for low to medium speed only. At high speed, instead, it’s totally normal that your body needs to relax its core to the advantage of performing faster repetitions, leading your posture to loose his core stiffness and consequently looking “weird” to the eye.
Stage 1: on knuckles (100 pushups in 1:09)
Stage 2: on hands (100 pushups in one minute)