So as you may have seen in the show, 9 chevron dialing has quite a few differences to standard 7 chevron or 8 chevron dialing. I spent a bit of time thinking about these differences, and decided to share my take on them.
- Everything becoming "projectiles" upon exiting a wormhole:
As you may have noticed, in SGU: Air, Part 1, people are thrown out of the Stargate at a higher velocity to that which they entered in at. This is also mentioned in Twin Destinies, where Telford states that "Everything we carry with us becomes a projectile in a nine-chevron transit".
I believe this may be due to some sort of "bodge" the Ancients implemented, rather than programming a permanent, safer solution.. Since the Stargate de-materialises the travelers, converting them to energy during the transfer through the wormhole. Therefore, the inertial properties of objects or people transferred would also be sent as data to the receiving Stargate. Given that a 9 chevron address will lead to a Stargate of an unspecified distance away (most likely a much further distance than 7 or 8 chevron addresses) the velocity properties of objects is purposefully disregarded, possibly in order to reduce potential interference to the matter stream during transfer, or to allow for the matter itself to be boosted in order to reach the destination. When a traveler arrives through a Stargate, the Stargate, having no idea what velocity the object or person should have, forcefully ejects them from the event horizon, in order to ensure that they leave the gate. Evidence of Stargates behaving this way was seen in SG1: Red Sky, where SG-1 was thrown out of the gate after certain protocols in the dialing sequence had been overridden. Possibly, the matter stream was interfered with during transit, causing the velocities of those arriving to be lost.
- Radio signals are unable to travel through a 9th chevron wormhole:
In Air, Part 1, Scott tries to radio through the wormhole to tell people to "slow down", as they are proceeding through the gate too fast. However, his message does not reach the other side. In Twin Destinies, Telford mentions "it seems that a radio signal does not reach across a nine-chevron connection."
My theory on this is similar to that on objects becoming projectiles in 9th chevron transit. Radio signals are purposefully blocked from being bi-directional in order to minimize potential interference with the matter stream.
- Wormhole is still able to connect, regardless of whether or not the receiving Stargate is in a state of motion when a connection is attempted to be established:
Twice, when Destiny is dialed with a 9 chevron address, the ship is in a state of motion, specifically FTL. Destiny has been shown to receive this connection, and drop out of FTL, presumably to make a stable connection. This is unable to occur when a standard 7 chevron address is used to try and connect to Destiny.
I believe that, because 9 chevron addresses have no relation to the Stargate's physical position in space, a different protocol is used for locking onto the other Stargate, presumably using the benefits of the non-relative 9 chevron address to achieve a connection regardless of whether or not the Stargate receiving is currently in motion.