Many fixed-gear bikes are equipped with "flip-flop" hubs, designed to accept sprockets on either side. This is usually because they don't know the technique of "walking" the wheel back and forth in the fork ends. I know, I've tried it.
So instead of changing both your cog and your chainring by a couple of teeth or so - you can make a bigger change to just either the cog or the chainring and get the same result (get the same gear inches).
The idea is that, most of the time you would ride the fixed gear, but if you found your self far from home and getting tired, or were in unusually hilly terrain, you would turn the wheel around and use the freewheel. I'd definitely recommend a selection of gears for different conditions.75 times around.
Now the chain tension should be better, but the wheel is no longer centered between the chainstays. Fitness plays a part too. This will actually tighten the chain a little bit more. Generally I would recommend only a one-tooth difference in this case. I've tried lower ratios where the moment I've started leg braking, I've lost traction in the rear end - which can be very disconcerting.. I'd put the smaller sprocket on the side without the lockring, because it's less likely to come unscrewed. Recommended cadences tend to be around 80-120 rpm.
Don't be a dick.
Any standard track hub can also be used with a single-speed freewheel just by leaving the lockring off.75. Legs are designed to move you. Brake shoes are cheaper to replace when they wear out than clutches are...0..
For fixie conversions, that have horizontal or diagonal dropouts instead of track fork ends, a further constraint may be finding a ratio that is compatible with the desired chain length and rear wheel position. can be run differently depending on rings and cogs. High stress repeated too many times leads to overuse injury, and will deplete rather than build muscle.
Track, from the bottom up would be something like 44/16 - 47/17 - 50/18 - 53/19
Going larger with the track crank is better, as in both discipline, bigger is better.