You said you slap it to your legs I think.. Try this, hold it with your one hand and slap it with your two-three fingers (with the other hand of course) from downside, just a little lower from the head. It’s a reflex and should happen to all. As any muscle in your body would tense if poked..
I do this during the exercises when my erection starts to fall.
Starting messurements: 6.0 BPEL , 4.80 MEG ---- June/15