the idea is simple. there needs to be thrust in prices before adx goes above any barrier or level say 25/10 or even 10/ The Di plus or Di minus should be above ADX. This indicates the change in direction or change in underlying price and obviously followed by ADX indicator which is dependent on user which level it exceed.
The ADX - Shows Trend Strength
The =/- Di show Thrust or reversal in prices.
it helps in entering the directional change in prices early rather than waiting for ADX