Concentration for partial dodging, like everything else, means that the AP cost of anything else you are doing in the round is doubled. So for example you would do a partial dodge and a 2 AP attack (costing 4 AP) which would be resolved at -50 (as a 2 AP attack).
If you are using the faster rounds optional rule, rather than going by phases, you can think of that as spending half your AP on the concentration action (e.g. partial dodge) and leaving the other half your AP (normally 2) for your other action.
