Hands closer together, slightly elevated and slightly internally rotated.
But in the comments, a "Sylvester Koh" (no idea who he is, but his comments sound informed and so I wanted to confirm or deny) agrees with a "Kirstine Vest Have" who says:
Funny. A study using EMG says the hands should be externally rotated for max activation of the serratus ant.. Not internally rotated.
(Perhaps this study, though it's for an unstable surface.)
Sylvester Koh also says:
If you [have] clinical reason in terms of muscle biomechanics and physiology, internal rotation will result in the recruitment of the pectoral muscles ( internal rotators) which defeat the purpose of this exercise. Push up plus serves to isolate serratus anterior muscle activation only and removes as much recruitment of pectorals as possible. Besides performing external rotation, at the shoulders, isometric shoulder horizontal abduction would further deactivate the pectoral muscle via reciprocal inhibition.
In summary, in order to perform the perfect push up plus exercise, there should be thoracic kyphosis, lower cervical flexion, lumbar flexion with posterior pelvic tilt, shoulder external rotation, and isometric shoulder horizontal abduction, in order to fully activate the serratus anterior muscles.
Place your palms on the floor in a standard push up position such that your palms are directly under your shoulders. Try to contract your posterior deltoids such that your wanna bring your hands away from one another. Next flex your neck gently and focus on protracting your scapular as much as position. I would recommend that your keep your scapular protracted for as long as you can rather that continuously move through the range of motion like in this video. Make sure that your abdominal muscles and buttock muscles are contracted fully throughout the exercise such that your lower back is flat out.
So my questions are who is right, and what is the best form? Thanks!