To enter it the plane needs to be just about stopped going vertical.
Enter a slight dive, pull the throttle to idle, pull the plane vertical. If it snaps out, enter the vertical portion with a little more speed. When the plane just about stops going up add power to try and catch it there. You'll figure out how much to add after a few tries. Also, at this point pay attention to what the plane wants to do as the power comes in. You may need right rudder. Once you know what its going to need you can input it before you see the plane veering off.
If you enter the vertical portion with power the plane will continue to climb until it runs out of speed. you will probably still be holding power in at that time and most likely be above a power setting that the plane needs for hovering.

Enter at idle and try to catch the plane with a power setting that will hang it there.