That is similar to ours. In the easiest example I can give, ours goes like this.
We are basically scheduled 1040 hours between Jan.1st to Jun.30
Of that 1040 hours you are scheduled, you must work at least 50% (520) of those hours in that six month time frame.
Same goes for Jul.1st to Dec.31st. 50% in that six month time frame.
Example A: My shift is 6:00-14:30 and I let someone work it for me...I am down 8 hours.
Example B: My shift is 6:00-14:30 and I give someone my shift and pick up a 14:00-22:30...nothing lost, nothing gained.
Example C: My shift is 6:00-14:30 and someone gives me their 14:30-23:00...I have picked up an extra 8 hours.
Bottom line, no matter how you do it in a six month period, as long as you worked 50% of the 1040 hours each six months, you're good.
Also let me add, if you are falling short on your goal of the 50% and cannot find anyone to work for, overtime hours DO count toward your 50% total hours. If you are held over four hours, those four hours count. (not the six you actually get paid)
Hope I have you all confused now...it really is simple.