Stroking the front brake lever is done during manual bleeding.
Resist the temptation to stroke or flick the lever to cause air bubbles to rise.
Air will rise in the master cylinder as can be seen from bubbles in the reservoir as the return stroke of the lever pulls fluid back into the master cylinder.
Air will try to follow fluid in it's return hence this occurence.
However, air can come up in the form of fine mist. And in vicous brake fluid, this mist takes some time to break surface tension and dissipate.
What will happen as you continue to manually bleed or stroke the lever is air mist travelling in and out of the master cylinder.
This will extend your brake bleeding time considerably. Do it firmly, smoothly.
Especially for ABS equipped motorcycles which requires more time to purge air in the ABS modulator, avoid this stroking/flicking lever action.