>>513916245
No. It's your reaction to the bullet that knocks you down. Been that way since this video was recorded in the 1980s.
https://www.youtube.com/watch?v=7QBjOio_sds
Now of course the NIJ does permit backface deformation up to just below 44mm in depth. That's not fun. However, a cracked rib will not reliably incapacitate someone. A bruise will not reliably incapacitate someone. Adrenaline and drugs will keep bad guys in play. You have to either hit the CNS or bleed them out. The former is faster than the latter.
>how many rounds to incapacitate
Depends entirely on the plate. Note that NIJ is NOT linear. Level III plates are rated to defeat 6 7.62x51 M80. Level IV plates at minimum must only defeat 1x .30-06 M2AP. They do not need to pass the Level III test.
Anons have tested very poor Level IV plates like the RMA 1155 and found they lose to .308 on the second hit. Better plates like the Hesco 4403 can handle at least three. Ditto for military ESAPIs or XSAPIs. Hesco 4601 can handle the full six.
For 5.56, double those numbers to account for lesser crack propagation across the ceramic strike face. Half a mag is not practical to best a Hesco 4601.
Now what you CAN do is concentrate hits. If rounds hit next to each other, they will only face already-compromised ceramic. For the 4601, you need to land hits within <2" of each other. Good luck.
Other than that, pierce with tougher ammo or practice pelvis shots. The latter is the most practical solution. Brute forcing armor is just clumsy.