Opposite Track Path

Opposite Track Path (OTP) is a technique used in optical technology, which when correctly implemented allows faster (though not un-noticeable) switching from layer 0 to layer 1.
A dual-layer DVD or Blu-ray with Opposite Track Path means that layer 0 is written in a spiral track starting at the inside of the disc reading outwards, whereas the layer 1 spiral starts at the outside of the disc reading inwards. Both tracks are written so that the DVD player will rotate the disc in the same direction. Unlike DVD-ROM discs, DVD video discs are mastered such that layer 0 ends at the same diameter as the layer 1 starts. This means that the layer 1 occupies the end of the layer rather than the start. Thus, although the laser still has to refocus on layer 1, it does not have to seek across the disc to find it.
With the DVD technology, a small gap is left in the layer 1 track every time it crosses the layer 0 track (twice per revolution). As a result, the data capacity of layer 1 is less than layer 0 which means that a dual layer disc only has a capacity of 8.5 GB instead of the 9.4 GB that might have been expected.
In the case of Blu-rays, it was realised that the gap was unnecessary as the laser is perfectly capable of focussing through track 0. Consequently, dual layer Blu-rays have double the capacity of their single layer counterparts (50 GB as against 25 GB).