How to Defend a Single Leg Takedown
The single leg takedown is a staple of Brazilian Jiu Jitsu. Originally appropriated from wrestling, the attack is a simple, direct way to bring your opponent to the ground, and a great way to land in a solid position from which to pass the guard (or straight into side control). As a consequence, your defense must be effective. Here are different approaches that draw upon Brazilian Jiu Jitsu, judo, and wrestling for answers on how to approach the single leg, along with universal principles.
When a partner picks up your leg in order to take you down—whether from a half guard position, or whether from the feet—you need to remember a few key things up front. First, it's much, much better to have your left leg (say) on your partner's left side (or in between your legs) than to have your leg on the other side, where they can easily finish a simple takedown. Second, it's important not to allow them to switch to a double leg takedown. This means keeping your other leg ("far leg") very far away, not to be grabbed by your partner. Once this baseline is established, reach with your left hand to your partner's jacket (right on top, just beyond the collar) and stiff-arm them to keep them away. While doing the stiffarm, be sure your leg is to your left of your partner, not as previously described, across their body. Finally, stomp the ground to free your trapped leg.
Sometimes, your partner is going to insist on keeping your leg in between their legs. This might allow them to better switch to a double leg, but more likely, they're interested in running the pipe (more on that later), or another follow up finish. Never fear; the initial response at your end is the same. First, be sure to grip your partner's jacket (just above the collar) to keep them from closing the distance too quickly. Next up, grip your partner's belt, but be sure you keep some downward pressure on your partner's shoulder, making sure they can't posture up (and lift you off the ground!). Now take control of your partner's far sleeve, helping to turn their shoulders toward you. It's time to hook their near-side leg with your trapped leg, making sure to create a platform of sorts with your thigh. This will then allow you to hop toward your partner's supporting leg, ultimately causing them to turn and fall with the resulting uchimata counter.
Similar to the uchimata, the sumi gaeshi counter is a natural follow up from judo. Start with the same initial conditions from the uchimata, but when you notice that your partner is unwilling to be thrown by the uchimata, switch directions by hopping underneath your partner. From here, throw them to the side and rear corner, taking some care to avoid pulling your partner directly across your face (unless, of course, you enjoy gi burn).
Drag Down and Crucifix
While you can also throw your partner with the sumi gaeshi while utilizing the Kimura grip, it's even easier (depending on their response, of course) to simply drape your body over your partner's back and kind of drag them to the ground. Use the same principle from the "stomp" move described earlier to keep your partner at bay initially, and then begin to move away from them. As they are forced to reach further and further (because you're doing such a good job of letting them close the gap, by controlling their gi above the lapels), it is relatively easy to simply drag them to the ground due to their horrible posture (which you've opportunistically created yourself). Once on the way down, try to hook an arm so that you can set up a crucifix.
Single or Double -- which is harder to defend?
Followers will notice that I resisted the temptation to include more Kimura related takedown defenses. As those already exist here, it seemed more prudent to spend time developing these somewhat more conventional options. There are many, many ways to deal with wrestling takedowns, especially in the gi (the great equalizer for takedowns in BJJ), and you will likely come across different options that work better for you than these described here. That's all right, too! Jiu jitsu has many, many possible solutions, and quite a lot of them are "correct." As always, please let me know if these techniques are working for you!