Time lag represent elapsed time, different to activities do not require updating, this happens automatically as soon as predecessor event happens.  Of course progressed lag is of interest and can be tricky when you have different lag calendars.

• http://www.ronwinterconsulting.com/Making_CPM_Transparent.pdf
• In-Progress Lag Report and Value. Remaining Lag should be displayed just as remaining duration is shown and editable. The CPM feature of Remaining Duration was added so that Schedulers could monitor and change this calculated duration result. Lags are to relationships what durations are to activities. Why should Remaining Lag calculations be discarded by the software instead of saved and displayed? Why should we be unable to indicate that actual remaining lag is only 2 days instead of the calculated 4 days? Part of the reason CPM Schedules are so hard to understand is that fact that not all of the data used is displayed.

Volume lag, not available in P6 also happens automatically as the work volume of predecessor activity progress.