Temporal mechanics can be summarised as follows:
Any given time traveller moving from one time to another at a rate greater than the local entropic norm (i.e. moving forward in time faster than the normal "subjective second"-to-"objective second" ratio), or in an reverse entropic vector (i.e. backwards in time), has to move outside of normal space-time. This movement was mathematically calcuated by a scientist named Feynman to be a predictable curve.
Thus, people displaced in time can use the data points of the "start" and "end" points of their journey to caclulate their return trajectory along their circumstance-specific Feynman Curve to return to their own time. It can also be used to calculate a path to initiate a controlled time displacement.
The return journey along that Feynman Curve is entirely dependent on the actions (or more accurately, the non-actions) of the displaced personnel. If their interaction is kept to a minimum and the integrity of the timestream is maintained, their Feynman Curve also retains its integrity and the displaced personnel can return to their own time in the same manner they were initially displaced, be that a slingshot trajectory around a star or using the Bajoran Orb of Time.
However, if the actions (or non-actions) of the displaced personnel cause a major disruption in the timestream, a new future is created from that point and the Feynman Curve of the displaced personnel collapses. This is because the Curve's end point--the traveller's start point--no longer exists, except in a now parallel reality completely disconnected from the timestream the traveller currently inhabits. An analogy of this is the severing one end of a rope bridge: The other side of the chasm is still there, but the traveller's connection to it is now lost, and they have to consider another means to get there--such as interdimensional travel.
The conditions and equations defining time travel are very selective and highly complex. For temporally
- This explanation is brought to you courtesy of Judith and Garfield Reeves-Stevens' book Millenium series Book 2 - The War of the Prophets.