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I've also observed this behaviour in friends, and was curious to see what research has been done on the topic. Here's what I found (summary at the end).

Sechrest and Flores (1971) study of leg-jiggling

Sechrest and Flores (1971) performed an observational study of the prevalence of leg-jiggling

leg jiggling was defined as a vertical, rhythmic movement of one or both legs while the subject was in a seated position

Their findings, albeit based on varying-quality data were that:

leg jiggling is more common in the Philippines than in the United States

individual differences exist in prevalence of leg jiggling

people may jiggle their legs more when alone than in company

leg jiggling appears to be more prevalent in males than in females

leg jiggling is facilitated by type of sitting position. Specifically, legs uncrossed makes leg jiggling a lot easier and more likely.

They tentatively concluded that leg jiggling is a symptom of tension and classified it as a nervous mannerism. They also suggested, given cultural differences, that it may be acquired through imitation of others.

Leg jiggling as communication

Smith and Naryan (2008), in their conference abstract, discuss features of leg jiggling and conclude that:

while so far we can only speculate about the causal and functional properties of jiggles, they are clearly substantially rule-governed, sensitive to both formal and semantic aspects of ongoing discourse, widespread - both patterns were observed in all discourse contexts, and both languages - and deserving of further attention.

Restless leg syndrome

There is a disorder called restless leg syndrome. From the description the leg movement is voluntary but the desire to move the leg sounds much stronger than what we are talking about with normal leg jiggling.

Fidgeting

Mehrabian and Friedman (1986) did a general study of fidgeting where leg jiggling was classified with a wide range of other fidgeting behaviours. Fidgeting was defined as:

engaging in manipulations of one's own body parts or other objects, such actions being peripheral or nonessential to central ongoing events or tasks

They developed a self-reported fidgeting scale and proceeded to correlate the scale with a range of other measures. Given the composite nature of the scale, it is difficult to say what correlates leg jiggling specifically has with other measures. And even if such correlations were known, it would only say something about what types of people jiggle their legs rather than the effect leg jiggling has for those that do jiggle.

Student concentration

Appleton (1969) in a study of student concentration suggests

Individual styles and habits reveal personal means of coping with the profound isolation of the concentrating state. Leg jiggling, for example, may be an outlet for sexual tension or a method of exercise and movement. Many students find the physical inactivity demanded by heavy work loads to be extremely difficult to bear.

Summary of links between leg jiggling and focus

So, in relation to your specific question on leg jiggling facilitating focus, the research that I've found is fairly limited. Researchers have noticed that leg jiggling is used by students when studying in order to deal with the otherwise, long periods of no movement. This is consistent with my own thoughts that it might be related to a general desire to increase circulation. Several researchers have also posited that leg jiggling may be a way of dealing with stress, which arguably might assist with concentration. But ultimately, it looks like more research is needed on this topic, particularly experimental evidence on the effects of leg jiggling.

References