Functional isometric training involves lifting a weight/resistance to a specific joint angle and holding in that position for 5-10 secs. This technique is often used to overcome the sticking point (the weakest point in the range of movement of an exercise) to increase maximal strength. You can also use this method for improving strength by performing the exercise at multiple joint angles.
What little research there is indicates that this technique may be useful for improving maximal leg strength but not as effective as traditional training for improving leg power. Many sports require leg power, and therefore you may want to consider it’s application if that’s your goal.
Typically you do functional isometric training with rack pins set at the desired joint angle. Then for the isometric portion of the lift, you push against the pins with maximum force using an empty barbell. Getting into and out of position can be very awkward. Especially if you don’t have a second set of pins to rest the bar on just under the pins you are pushing against.
Another variation of functional isometric training is to set the pins 1-2 inches below the desired joint angle and lift the barbell just off the pins and hold in position. With this method, you can load up the bar to the maximum you can hold for 5-10 secs and progress from there.
There are a few proponents of isometric training. But when you’re not moving, and the bar isn’t going anywhere, it’s hard to get too motivated by it.