I find using a straight arm forehand that: 1. the arm goes straight...2. the wrist is relaxed and lags...3. using almost totally relaxation at
contact point natural body kinetics is for the elbow to start to bend (not with the upper arm moving higher or the elbow popping up like you see some do) along with pronation of the forearm and wrist...4. this elbow bend and forearm wrist pronation accelerates the racquet head to brush up faster on the ball and finish the follow through of the stroke.
My comment is with total relaxation shadow swing paying attention to how the body operates and feels (slow motion) to function naturally and protect itself (don't force the motion). You will find the elbow will automatically bend as the hand can no longer move horizontal towards the net. Plus, when swinging from the shoulder the racquet path is \ shaped, low to high.
Like when serving if the proper amount of shoulder tilt for the trophy position the racquet path is vertical towards the sky and the forearm and wrist pronate naturally to protect the shoulder. Thus, the comment, "hit up", trying to keep you from swing towards the net with shoulders parallel to the court surface. Explained from a right-handed perspective, using a pinpoint motion as the back right foot steps forward towards the heel of the front left foot (don't step around the front foot, stay behind the front foot) really bend the right knee applying most of your weight to the right leg. (Almost like when you were a kid doing a cartwheel on the grass in the front yard.) Now from this position notice how much your shoulders are tilted, then swing up, pushing with the right leg aggressively. The arm will go Up not towards the net and the forearm and wrist at the top of the up swing will pronate to protect the shoulder. This is how you can keep from hitting serves long or in the net. Great trophy position (shoulder tilt) and swing racquet UP UP UP towards the sky.
Now use this same type of natural relaxed body kinetics for the forehand, quit arming the ball, the arm is useless for true power it's
weak in comparison to the legs and core. However, the arm and wrist control directional consistency, fine tuning your accurate return
of the ball where you want it to go.
So, RELAX, let it happen, feel what happens, make small adjustments, swing out, learn and have more fun. Also, quit trying to win so
much, winning happens naturally after developing consistent control of power.
Aloha