Larsen Variation (4...dxe5 5.Nxe5 c6)

The idea

After 4...dxe5 5.Nxe5 c6, Black has removed the advanced pawn, and White's knight sits on e5. Wikipedia says 5...c6 has become more common than 5...g6, and in top-level chess has largely displaced 4...Bg4 as the main line.

Plans for White

Use the active knight and the space: Be2 or Bc4, castle, and c4 to push the knight on d5 back. White's pieces are more active, and the plan is to keep the knight on e5 strong.

Plans for Black

Develop with ...Bf5 or ...Nd7 to challenge the knight on e5, then ...e6 and castle. The ...c6 move supports the knight on d5 and discourages White's most ambitious continuations, as Wikipedia notes.

Watch out for

For Black, f7. With White's knight on e5, Bc4 or Qf3 can aim at f7. Larsen lost quickly to Fischer after the "suicidal" 8...Kd6 in a sacrificial line of this variation, as Wikipedia puts it.

For White, the knight on e5. If it's exchanged or driven back, White's lead in activity disappears.