The Apple watch App is the second screen and the remote control for the Tracking. Therefore it is important that the iPhone is near to the watch and activated.
The App must be activated before you start tracking, so the exact GPS position can be determined and transmitted. The GPS location is used in the background of the App and doesn't use any extra battery.
To save you track without pictures via the iSKI Apple Watch App your iPhone has to be in close proximity to your Apple Watch, in order for it to save and analyse the data properly.