In the Sound Of My Voice, in the last scene we see Maggie does a secret handshake with the kidnapped girl, and the kidnapped girl says:
How do you know my secret handshake?
However, how can someone develop a secret handshake without a partner to do it with? The kidnapped girl is obviously alone throughout the entire movie, and doesn't have someone to create a secret handshake with.
Abigail is potentially autistic and the handshake may be therapeutic
While not called out specifically, Abigail shows behavior which may indicate some level of autism. A good summary is from this review:
At one point, Maggie says she needs Peter to prove himself by bringing her a young girl, Abigail (Avery Kristen Pohl) from the school Peter substitute teaches at. This little girl exemplifies autistic-like tendencies: she's withdrawn, does not really speak, and focuses on building extremely detailed lego structures.
Emphasis mine. Large portions of Abigail's background are probably intentionally left out of the movie, so that we aren't given any convenience to deny Maggie's claims. However, Abigail may have been instructed to come up with a secret handshake by a social worker, therapist or even her father. Shaking hands and saying goodbye is not necessarily a simple thing for a person on the autistic spectrum.
The most specific example I found is only surviving right now is cached on Google, in a list of tips for parents dealing with the autistic kids and school:
- Develop a set of rituals, for example: (a) a “chit-chat” ritual at bedtime (e.g., ask your youngster about the happy, sad, scary and frustrating parts to his or her day; (b) a goodbye ritual (e.g., develop a secret handshake with your youngster that’s used only when he or she leaves you; (c) an after-school ritual (e.g., let your youngster have a snack and play outside for 30 minutes before starting homework); (d) an end-of-the-week ritual (e.g., have a “family night” every Saturday to reconnect and unwind after a busy week).
So it might not have been something developed with a friend, it could be her father or someone else in her support network.
That's one of the main enigmas of the movie. Even if you assume that Maggie is lying (and that she's not from the future) then how did she know the handshake?
The simplest (occam's razor) answer is that her Abigail's father was somehow linked to the cult, potentially providing video footage or at least a description. In a much earlier draft, the cult was characterised by the use of the internet and webcams (the cult itself was going to be named after an IP address) so that could also offer a possibility of how they came across web-footage of the handshake.
This interview makes it clear that the ending was deliberately vague. You're intended to come to your own conclusion based on your personal prejudices about religion and science
As to the handshake itself, I see no special reason why you couldn't invent it on your own;
