In the non-linear analysis the user specifies "load steps".  The total load is divided by the number of load steps and then applied.  For example, if you applied 100 lbs total wind load to a stack, and you had 10 load steps, then 10 lbs is applied each load step.  After each load step the deflections of the structure are calculated.  If the load case has dead load then these dead loads are being applied to joints which are deformed relative to their original position, and so that creates a secondary moment.  This effect is how we simulate the P-Delta effect, so the more load steps the more accurate the P-Delta effect.  

If you want to compare the P-Delta effect, for a self supported stack, then you would run the analysis using the Linear and then the Non-linear option and compare the results.

Please note that the Non-linear analysis is only available in the Pro version of MecaStack.