I am mostly speaking from a belly flying standpoint (as it seems the thread mostly pertains to) as vertical flying is another discussion. While I agree with others that there is some variable that is determined mostly by the organizer and/or person giving the count as to what has worked best in their (hopefully) vast experience and what they are comfortable with (IMO much more important then one or more people not giving the count). Think about it, if you are use to a particular count and all you have to do is leave at the right time, it is much harder for the person giving the count to change the routine that may be just as ingrained. Some of the variables being the size of actual chunk/formation being launched vs people inside/outside the door leaving on the count but not interfering with the launched formation, all jumpers experience, etc. In 4-way the shake is effective more as a mindset and is not necessarily part of the count and deserves a discussion of its own so in this reply I am mostly thinking of larger launched groups where a round is likely to be the launched formation... If you are launching a 8-way springbok you probably don't need this discussion! IMO a shake is a little ridiculous as it cannot be conveyed effectively on the outside of a plane where everyone is moving to get into position. The outside center (OC) is the best place for the count to come from as they are best seen by everyone. They are also the last to get out so everyone else should be getting settled. The OC looks to the rear and establishes eye contact, next to the front again with eye contact. It is noted that when you establish eye contact it means you are ready, do not look to the OC until you are. Once contact is made you are responsible for staying "ready". Last the OC looks inside for eye contact (which are the last to move into position) and then starts the count. It begins with OC slight pause to settle and reset. The count needs to be seen by everyone and it is a best practice to preload the weight of the leg pushing off on "set" (that goes for everyone on every launch). Again this depends on person giving the count but I have found a large head movement downward (like a big head nod) for ready, then left trailing leg (weight on right leg with body facing realitive wind) large knee movement up almost like kneeing the inside center and head moving up at the same time for set and obviously leaving the plane on go. The count is given at a consistent brisk but not fast pace as this would minimize time allowed for leaving early or late (I know you are now trying to chime in about floaters but that is yet again for another discussion). Ready, set, go is also communicated verbally but is not necessarily heard by everyone. If this is long winded you probably are not taking enough time to mock and prepare your launch. Let's not forget that it is the biggest thing that will determine the rest of the skydives success! The next discussion should be everyone's job leaving the plane for a successful launch.
