You need a handle. Make the dividing fence and stretch a tape with the handle on it between two of the posts (put them about 6ft apart) Then I just make a loop in the end of the actual fence tape and put the handle hook through the loop.
Goodness, that sounds like rubbish but I can't describe it better!
if you dont want the electric to sag round the field every time you open the gate (relax the tension between the posts) then you need to get some zip ties and zip tie the tape to the last post before the bit that you will use as the gate.