Toes pointed, ankle rigid: only a locked foot refuses to give way on contact. A soft ankle swallows exactly the energy that was meant for the ball.
How hard you strike a ball comes down to two things: how fast the foot is and how solid the contact is. An instep drive sends the ball out at roughly 18 to 35 m/s depending on level, age and approach angle — but only if the foot does not give on impact. That makes the locked ankle the cheapest upgrade available to you: toes down, instep firm, and tensed before contact rather than during it. The classic fault is the toe-poke, where the foot folds back on impact and the power simply leaks away. The same principle governs the inside-foot pass: let the ankle wobble and your ground pass lifts. One caveat belongs with it — maximum ball speed is not the point. Aim deliberately and the whole movement slows down, and a placed ball counts for more than a rocket past the post.