One of the best golf swing tips is to keep your left knee flexed until your club face has contacted your ball on your through swing.
The reason why this is one of the best golf swing tips is that if you straighten your left leg and lock your left knee joint before your clubface contacts your ball you will most likely hit a push-slice golf shot.
A push-slice golf shot resembles a shank and is likewise as ugly. This is no way to hit a golf ball straight.
If you listen to some of the leading golf analysts on television nowadays you may come to believe that you should straighten your left leg as you deliver your clubface to your ball at impact.
Some analysts imply such when they speak about the importance of hitting against a firm left side. Indeed, some golf swing instructions even talk about straightening the left leg when the clubface is delivered to the ball.
I do not believe the intent of their comments is to suggest you should straighten your left leg and lock your left knee joint as you deliver your clubface to your ball at impact but, from their comments about hitting against a firm left side, you may have come to such conclusion.
My take is their golf swing instructions are to suggest that you should straighten your left leg immediately after impact, not at impact.
Sometimes I observe golfers on a practice range lock their left knee joint as they deliver their clubface to their ball at impact and nothing but horrific golf shots result.
Therefore, irrespective of what you may have interpreted from listening to golf analysts about hitting against a firm left side, please do not conclude they mean to imply you should lock your left knee joint as you deliver your clubface to your ball at impact.
For those who have misinterpreted such comments from golf analysts and have incorporated the notion that they should straighten their left leg at impact, this may one of the best golf swing tips they have ever heard.