This
is a great, simple-equipment exercise for the triceps. It's
one of my favorite tricep extension type of exercises because
I find it to be the easiest on the elbows due to a couple
of factors...first, because you're using dumbells and second,
because you're holding the dumbells with a neutral grip
(more below on that).
I prefer
to do this exercise laying cross-bench, but it can be done
on the floor or laying "normal" lengthwise on
the bench, too. I like to perform it cross-bench, but it's
mostly just personal preference. It just feels better to
me that way. You can do it any of the ways and get great
results.
So first,
grab two dumbells (same weight on each one!) then lay back
on the bench. In the video, you'll see how I get into position
with the cross-bench technique. I basically squat down in
front of the bench then flop back down onto it (only do
THAT if your gym floor isn't slippery and the bench won't
slide away from you). If your floor is slippery, use the
bench lengthwise.

Now
lay back on the bench and hold the dumbells up at arms-length
above you. Your palms should be facing in towards each other
to put your hands in the neutral position I mentioned above.

Now,
keeping your upper arms as vertical as possible, lower the
dumbells down beside your head.

Extend
the triceps and bring the dumbells back up and you've done
a rep! When you've done as many reps as you can, it's time
to set the dumbells down.
When
I do the cross-bench technique, I lay down towards one end
of the bench. At the end of the set, I just lower the one
dumbell out to the side and set it on the bench. Then I
can more easily get the other dumbell down to the floor
because I have two hands to do it with. This comes in handy
when you're trashed at the end of a set!

Give
this one a try in your next tricep workout! It's a good
one - remember, you can do it lying on the floor or lengthwise
on the bench too, if it doesn't feel good to you going cross-bench.
I like
this one because it allows me to do a tricep extension movement
without having to take the time to load a barbell, which
can be critical when you're taking very short rest periods
and need all the rest you can get!