in order too hold back you simply apply a little friction to the line leaving the spool by gentle finger pressure applied to the spools rim /front of spool effectivley slowing the line's release ,and retarding the floats smooth progress in doing so.
a lot depends on the reel your going to use ,a centrepin would only require the gentle touch of thumb on its rim
a closedfaced reel you'd cover the gap between the cowling and winding cup / relaese button and trap (& release) the line behind your finger or let it trickle over it
with a openfaced /fixed spoolled reel you trap (& release) the line behind your finger pressed against the front rim of the spool oe direct too the line on the spool& let it tricklepast it slowing it down .
when your float is running through your swim ,the float is actually travelling slightly faster than your bait near the bottom and lags behind the line
but when you apply pressure to the line you slow the float ,allowing the bait to precead the line in the flow .
and the harder you hold back the higher the bait rides up in the current ,which the fish can find irrisistable as it flutters in the current