I've been riding a fixed-gear bicycle since the third week
of August. Every time I ride one of my derailer bikes, the
bottom bracket feels (very subjectively) as though it is
almost seized. I've become used to being encouraged
by the pedal upstroke pushing on the bottom of each foot,
and when it is not there, the bike feels like it is in a kind
of mechanical failure. The first time this happened, the
bike felt like it was out of control (couldn't turn the pedals,
even though it was just my mind -- and muscle memory --
in play).

I ordinarily do not use the hand brakes, and I do not run
any foot retention system.

One time (very recently) I forgot what I was riding, and
sprinted after someone, and lost control (some part of
me expected to be able to freewheel after the sprint) --
my feet left the pedals, for at least half a rotation of the
bottom bracket spindle (I have no idea just how long my
feet were entirely off the pedals).

I recovered fine (never touched down or dismounted).
I'm still far from convinced that binding my feet to the
pedals would have resulted in a safer outcome -- my
body knew what to do, to get back onto the pedals (and
it also managed to stay in the saddle during the interim).

A coach, physically present and riding alongside, is to
me the obvious next step, but that's unlikely; so I will
probably soon invest in some PowerGrips type of pedal
straps. I was going to make my own, but thought that
this was not the time to get one of my bright ideas about
things.