The ability to lock the rear wheel by reverse pedal pressure depends on your strength and the gear ratio.
Leaning forward makes it easier by shifting weight off the rear wheel but is counter productive if your intent is to sop the bike rather than simply skid to show you can do it.
I suggest you switch sprockets to a lower gear (larger rear sprocket), to find a balance between control, acceleration and the ability to lock or slow the rear wheel (improves with lower gear), and the ability to cruise at higher speeds without your legs spinning faster than you can manage (higher gear helps).
Once you have than, then your goal is to practice short stops (not just skids) by resisting pedal motion, and short pulsed skids each time the crank are horizontal. You want to be able to do this without shifting weight forward more than the braking alone causes. In fact you want to be able to do this with weight shifted back to get the shortest most controlled stop.
BTW- since you mentioned it, chain tension has nothing to do with this, and the correct tension is still NO tension, with the chain slightly slack (sags about 1/4" in the middle).
Now go find a parking lot and practice.
__________________
FB
Chain-L site
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