Gracie Barra Bristol, (BJJ), Can Sönmez, Bristol, UK - 19/04/2012
I've reached the milestone of my fiftieth class as an instructor: pleasingly, it's one I'm comfortable teaching, as I went through my favourite mount escape, the heel drag. It is also quite simple, which is another reason I like it. You're in mount, your elbows in a good place for defence, down by their knees. For this escape to work, you need to have one of your legs out flat. You also need to get on your side: a slight bridging motion will help.
The big danger at this point is that the person on top will switch to technical mount. You therefore need to make sure that your neck is safe if that happens. You also don't want to let them settle into technical mount: immediately prepare your frames to start escaping before they secure the position. You may even be able to disrupt them as they try to shift, using that shift in their base to enter into your escape.
If they don't get to technical mount, or you're able to work back to the previous position, wedge an elbow underneath their knee. You can either make a frame against their hips, or if you're concerned about your neck, adjust so that you can still pry your elbow under their knee while protecting your collar with your hands. As well as chokes, you also need to be wary of their cross-face: if they can control your head, they can flatten you back out, which will make the escape less effective. Use a combination of your elbow and shrimping to shove their knee backwards, on your flat leg side.
Bring your other foot over both your flat leg and the leg they have next to it. That means you can use the heel of that foot to drag their leg over your flat leg. As soon as you get it over, lock half guard and shrimp towards their trapped leg. In half guard, you want to get onto your side as quickly as possible: if you stay flat on your back, you've already done their work for them, as they will want to flatten you out in order to pass half guard. If you're comfortable in half guard, you could stay there and work your attacks.
Alternatively, keep shrimping in the other direction, in order to free your other leg, just like you would with an elbow escape. It's also worth noting that some people, like Roy Dean, recommend just pinching your knees rather than fully triangling your legs around theirs, so that's worth trying too. To help recover full guard, you can also bring your arm across to their opposite shoulder, impeding their movement while aiding yours. Emily Kwok has a handy tip too: if their foot is too flat, making it hard to get your heel in for a drag, shove under their heel with your knee to pry it up and create that space between their foot and the mat.
A very similar escape, which I don't use much, is the foot lift. Dean shows these two escapes in sequence on his awesome Blue Belt Requirements. The foot lift is for when they have some space underneath their in-step. People won't often do that, in my experience, but if they do, this time just step over your flat leg. Use your foot to hook underneath their instep and lift it over, then as before lock up half guard (your legs are already in position), or shrimp to recover full guard.
Make sure that you pay particular attention to shoving on their knee with this variation, as it is easier for them to slip free (though if that happens, you can always switch to the heel drag). With both escapes, it is important to get the knee of their trapped leg back behind your legs. If they still have their knee past your legs, it makes it much easier for them to move straight into a half guard pass, by driving their knee to the mat and sliding through.