Chain command blocks activate when an impulse command block running in from the side triggers (so a direction 90° different than the direction of the chain command block).

This can be replicated by:

1. Placing a chain command block with any command.
2. Place an impulse command block, whose direction isn't equal to the CCB's direction, but runs in from the side.
3. Give it any command.
4. Fire the impulse command block.

When this happens, you should have both command blocks fire, despite the ICB not going in the same direction.