There is no more humbling hamstring exercise than the Glute/Ham
Raise.

Seriously. Very few people can even do one complete rep.
Luckily, quarter reps and half reps aren't a waste of time as
even a limited range of motion on this movement will help you build
the knee flexion function of the hamstrings like no other hamstring
movement.

In addition to building up those hammies, it can also make an
athlete virtually invulnerable to hamstring injuries as the
movement lengthens the sarcomeres to an unparalleled
degree.

The goal is to get down on your knees and try to lower your
torso to the ground under control, then bring yourself back up. As
mentioned, very few athletes can actually bring themselves up at
first, so you might want to help yourself with a little arm push to
get you started off the ground.

Starting position: Kneel down and lock your feet under something
solid and heavy (a partner can do just fine, but he must be able to
hold you down). The trunk is upright and the arms are alongside the
body.

Execution: Lower yourself as slowly as possible. To do so you
must produce a powerful hamstring contraction or else you'll find
yourself embedded in the floor! If you're able to bring yourself
back up on your own, do so, but as mentioned, most guys will need a
little push-off with the arms to get moving.

An earlier, unevolved Christian Thibaudeau demonstrates the
glute/ham raise.

Remember to go easy on this movement until you develop some
strength and proficiency. In fact, you might want to do this
movement last in your workout to ensure that your hamstrings are
fully warmed (even though your strength will be at its
lowest).

Try doing as many reps as possible, making sure to stop before
your hamstrings rupture and you start screaming like a little girl
who just sat down on a hot stove.