Danaher says:

When attacking the legs – try to control all three joints: One of the reasons I favor leg lock attacks is because of the very robust connection you can make to an opponents body that makes it rather difficult for an opponent to pull away from you once the attack is launched. This is particularly true when you are able to connect to all three joints of the leg – hip, knee and ankle. You have a choice as which of those joints you seek to connect to first. Sometimes it’s good to begin with the heel/ankle (end of the lever). Sometimes knee and hip is good (keeps the opponents knee inside your knee line). But at some point, if resistance is strong and there is a danger of the opponent pulling completely away from you, it’s wise to find a strong connection to all three joints. Here, one hand controls the ankle, the other the knee and my feet and knees control the hip. Once the control has been established – then you can transition to submission – in this case it will require a movement to get my wrist under the heel – but it will be made considerably easier due to that initial three aspect control. The leg is a long and strong limb – maximize your chances with this principle of control.